实现基本关系代数运算，JOIN跨表查询

README.md

@@ -10,8 +10,6 @@
 | `chusql-storage/` | Rust    | 存储引擎：磁盘IO、B+树、缓冲池、WAL、崩溃恢复、并发控制           |
 | `chusql-web/`     | 前端    | 浏览器端管理客户端                                                |
 
-详细架构、模块职责与构建目标见 [ARCHITECTURE.md](ARCHITECTURE.md)。
-
 # 架构分层
 
 ## Haskell部分

chusql-engine/app/Main.hs

@@ -1,9 +1,9 @@
 module Main where
 
-import ChuSQL.Eval.Executor
+import ChuSQL.SQLSyntax.Executor
 import ChuSQL.Model
-import ChuSQL.Syntax.Ast
+import ChuSQL.SQLSyntax.AST
-import ChuSQL.Syntax.Parser
+import ChuSQL.SQLSyntax.Parser
 
 users :: Table
 users =

chusql-engine/chusql-engine.cabal

@@ -9,12 +9,15 @@ build-type:     Simple
 
 library
     hs-source-dirs:   src
-    exposed-modules:  ChuSQL.Syntax.Ast
+    exposed-modules:  ChuSQL.Algebra.Eval
-                      ChuSQL.Syntax.Parser
+                      ChuSQL.Algebra.Op
+                      ChuSQL.Algebra.Planner
                       ChuSQL.Model
-                      ChuSQL.Eval.Expr
+                      ChuSQL.SQLSyntax.AST
-                      ChuSQL.Eval.Sort
+                      ChuSQL.SQLSyntax.Executor
-                      ChuSQL.Eval.Executor
+                      ChuSQL.SQLSyntax.Expr
+                      ChuSQL.SQLSyntax.Parser
+                      ChuSQL.SQLSyntax.Sort
     build-depends:    base >= 4.14 && < 5
                     , megaparsec
                     , parser-combinators

chusql-engine/src/ChuSQL/Algebra/Eval.hs

@@ -0,0 +1,39 @@
+module ChuSQL.Algebra.Eval where
+
+import ChuSQL.Algebra.Op
+import ChuSQL.Model
+import ChuSQL.SQLSyntax.Expr (evalCondForRow)
+import ChuSQL.SQLSyntax.Sort (sortRows)
+import Control.Monad (filterM)
+
+-- 投影
+project :: [String] -> Row -> Row
+project ["*"] row = row
+project cs row = [(c, v) | (c, v) <- row, c `elem` cs]
+
+-- 执行关系代数运算
+addPrefix :: String -> Row -> Row
+addPrefix prefix = map (\(k, v) -> (prefix ++ k, v))
+
+evalRelOp :: Database -> RelOp -> Either String [Row]
+evalRelOp db (Scan mAlias tbl) = do
+    table <- maybe (Left ("unknown table: " ++ tbl)) Right (lookup tbl db)
+    let prefix = maybe "" (++ ".") mAlias
+    Right (map (addPrefix prefix) (tableRows table))
+evalRelOp db (Filter e op) = do
+    rows <- evalRelOp db op
+    filterM (evalCondForRow e) rows
+evalRelOp db (Project cols op) = do
+    rows <- evalRelOp db op
+    Right (map (project cols) rows)
+evalRelOp db (Sort spec op) = do
+    rows <- evalRelOp db op
+    Right (sortRows spec rows)
+evalRelOp db (Limit n op) = do
+    rows <- evalRelOp db op
+    Right (take n rows)
+evalRelOp db (Join left right cond) = do
+    lrows <- evalRelOp db left
+    rrows <- evalRelOp db right
+    let cross = [l ++ r | l <- lrows, r <- rrows] -- 笛卡尔积
+    filterM (evalCondForRow cond) cross

chusql-engine/src/ChuSQL/Algebra/Op.hs

@@ -0,0 +1,13 @@
+module ChuSQL.Algebra.Op where
+
+import ChuSQL.SQLSyntax.AST (Expr, SortDir)
+
+-- 关系代数算子
+data RelOp
+    = Scan (Maybe String) String
+    | Filter Expr RelOp
+    | Project [String] RelOp
+    | Sort [(String, SortDir)] RelOp
+    | Limit Int RelOp
+    | Join RelOp RelOp Expr
+    deriving (Show, Eq)

chusql-engine/src/ChuSQL/Algebra/Planner.hs

@@ -0,0 +1,73 @@
+module ChuSQL.Algebra.Planner where
+
+import ChuSQL.Algebra.Op
+import ChuSQL.Model
+import ChuSQL.SQLSyntax.AST
+
+-- * 存在性检查
+checkColumns :: [String] -> [String] -> Either String ()
+checkColumns available requested =
+    case [c | c <- requested, c /= "*", c `notElem` available] of
+        [] -> Right ()
+        (c : _) -> Left ("unknown column: " ++ c)
+
+checkFrom :: Database -> FromClause -> Either String ()
+checkFrom db (FromTable _ tbl) =
+    case lookup tbl db of
+        Just _ -> Right ()
+        Nothing -> Left ("unknown table: " ++ tbl)
+checkFrom db (FromJoin left _ tbl _) = do
+    _ <- checkFrom db left
+    case lookup tbl db of
+        Just _ -> Right ()
+        Nothing -> Left ("unknown table: " ++ tbl)
+
+-- | 收集 FromClause 里所有可用的列名
+availableColumns :: Database -> FromClause -> Either String [String]
+availableColumns db (FromTable mAlias tbl) = do
+    table <- maybe (Left ("unknown table: " ++ tbl)) Right (lookup tbl db)
+    let prefix = maybe "" (++ ".") mAlias
+    Right (map (prefix ++) (tableCols table))
+availableColumns db (FromJoin left mAlias tbl _) = do
+    lcols <- availableColumns db left
+    table <- maybe (Left ("unknown table: " ++ tbl)) Right (lookup tbl db)
+    let prefix = maybe "" (++ ".") mAlias
+    Right (lcols ++ map (prefix ++) (tableCols table))
+
+-- | 检查数据库列明
+checkColumnsFrom :: Database -> FromClause -> [String] -> Either String ()
+checkColumnsFrom db fromC requested = do
+    available <- availableColumns db fromC
+    checkColumns available requested
+
+-- 内部查询语句转执行计划
+fromToRelOp :: FromClause -> RelOp
+fromToRelOp (FromTable mAlias tbl) = Scan mAlias tbl
+fromToRelOp (FromJoin left mAlias tbl cond) =
+    Join (fromToRelOp left) (Scan mAlias tbl) cond
+
+translate :: Database -> Query -> Either String RelOp
+translate db q = case q of
+    Select
+        { selectCols = cols
+        , selectFrom = fromC
+        , selectWhere = mWhere
+        , selectOrderBy = orderBy
+        , selectLimit = mLimit
+        } -> do
+            -- 检查所有表和列
+            _ <- checkFrom db fromC
+            _ <- checkColumnsFrom db fromC cols
+            _ <- checkColumnsFrom db fromC (map fst orderBy)
+            let base = fromToRelOp fromC
+                filtered = case mWhere of
+                    Nothing -> base
+                    Just w -> Filter w base
+                sorted = case orderBy of
+                    [] -> filtered
+                    spec -> Sort spec filtered
+                limited = case mLimit of
+                    Nothing -> sorted
+                    Just n -> Limit n sorted
+            Right (Project cols limited)
+    _ -> Left "only SELECT is supported by translate"

chusql-engine/src/ChuSQL/SQLSyntax/AST.hs

@@ -1,10 +1,10 @@
-module ChuSQL.Syntax.Ast where
+module ChuSQL.SQLSyntax.AST where
 
 -- 查询语句
 data Query
     = Select
         { selectCols :: [String]
-        , selectTable :: String
+        , selectFrom :: FromClause
         , selectWhere :: Maybe Expr
         , selectOrderBy :: [(String, SortDir)]
         , selectLimit :: Maybe Int
@@ -14,6 +14,12 @@ data Query
     | Update String [(String, Expr)] (Maybe Expr)
     deriving (Show, Eq)
 
+-- 从句
+data FromClause
+    = FromTable (Maybe String) String
+    | FromJoin FromClause (Maybe String) String Expr
+    deriving (Show, Eq)
+
 -- 条件表达式
 data Expr
     = Col String
@@ -35,7 +41,7 @@ makeSelect :: [String] -> String -> Maybe Expr -> Query
 makeSelect cols tbl w =
     Select
         { selectCols = cols
-        , selectTable = tbl
+        , selectFrom = FromTable Nothing tbl
         , selectWhere = w
         , selectOrderBy = []
         , selectLimit = Nothing

chusql-engine/src/ChuSQL/SQLSyntax/Executor.hs

@@ -1,16 +1,12 @@
-module ChuSQL.Eval.Executor where
+module ChuSQL.SQLSyntax.Executor where
 
-import ChuSQL.Eval.Expr
+import ChuSQL.Algebra.Eval (evalRelOp)
-import ChuSQL.Eval.Sort
+import ChuSQL.Algebra.Planner (translate)
 import ChuSQL.Model
-import ChuSQL.Syntax.Ast
+import ChuSQL.SQLSyntax.AST
+import ChuSQL.SQLSyntax.Expr (evalCondForRow, evalExpr)
 import Control.Monad (filterM)
 
--- | 带限制的排序
-applyLimit :: Maybe Int -> [Row] -> [Row]
-applyLimit Nothing rows = rows
-applyLimit (Just n) rows = take n rows
-
 -- 表里加新行
 updateTable :: String -> (Table -> Table) -> Database -> Either String Database
 updateTable name f db = do
@@ -30,28 +26,10 @@ applyUpdates ((col, e) : rest) row = do
 
 -- 处理查询
 runQuery :: Database -> Query -> Either String (Database, [Row])
-runQuery
+runQuery db q@Select{} = do
-    db
+    relOp <- translate db q
-    Select
+    rows <- evalRelOp db relOp
-        { selectCols = cols
+    Right (db, rows)
-        , selectTable = tbl
-        , selectWhere = mWhere
-        , selectOrderBy = orderBy
-        , selectLimit = mLimit
-        } = do
-        table <- maybe (Left ("unknown table: " ++ tbl)) Right (lookup tbl db)
-        _ <- checkColumns (tableCols table) cols
-        _ <- checkColumns (tableCols table) (map fst orderBy)
-        rows <- case mWhere of
-            Nothing -> Right (tableRows table)
-            Just e -> filterM (evalCondForRow e) (tableRows table)
-        let sorted = sortRows orderBy rows
-            limited = applyLimit mLimit sorted
-        Right (db, map (project cols) limited)
-      where
-        project :: [String] -> Row -> Row
-        project ["*"] row = row
-        project cs row = [(c, v) | (c, v) <- row, c `elem` cs]
 
 -- 处理插入
 runQuery db (Insert tbl cols vals) = do
@@ -95,10 +73,3 @@ runQuery db (Update tbl assigns mWhere) = do
         if keep
             then applyUpdates asgns row
             else Right row
-
--- 显式请求的列必须存在（@*@ 表示全部列），避免静默返回一堆空行
-checkColumns :: [String] -> [String] -> Either String ()
-checkColumns available requested =
-    case [c | c <- requested, c /= "*", c `notElem` available] of
-        [] -> Right ()
-        (c : _) -> Left ("unknown column: " ++ c)

chusql-engine/src/ChuSQL/SQLSyntax/Expr.hs

@@ -1,7 +1,7 @@
-module ChuSQL.Eval.Expr where
+module ChuSQL.SQLSyntax.Expr where
 
 import ChuSQL.Model
-import ChuSQL.Syntax.Ast
+import ChuSQL.SQLSyntax.AST
 import Control.Monad (join)
 
 -- 表达式求值

chusql-engine/src/ChuSQL/SQLSyntax/Parser.hs

@@ -1,11 +1,11 @@
 {-# LANGUAGE ImportQualifiedPost #-}
 
-{- | SQL 语法分析器：把 SQL 文本解析为 'ChuSQL.Syntax.Ast.Query'。
+{- | SQL 语法分析器：把 SQL 文本解析为 'ChuSQL.SQLSyntax.AST.Query'。
 
 词法层负责空白、关键字、标识符、字面量与符号；语法层按
 优先级 比较 > AND > OR 自底向上组装表达式。
 -}
-module ChuSQL.Syntax.Parser (
+module ChuSQL.SQLSyntax.Parser (
     Parser,
     parseQuery,
     expr,
@@ -14,9 +14,10 @@ module ChuSQL.Syntax.Parser (
     stringLit,
 ) where
 
-import ChuSQL.Syntax.Ast
+import ChuSQL.SQLSyntax.AST
 import Control.Monad.Combinators.Expr (Operator (..), makeExprParser)
-import Data.Char (isAlpha, isAlphaNum)
+import Data.Char (isAlpha, isAlphaNum, toLower)
+import Data.List (intercalate)
 import Data.Maybe (fromMaybe)
 import Data.Void (Void)
 import Text.Megaparsec
@@ -88,6 +89,13 @@ limitClause = do
     keyword "limit"
     integer
 
+-- | 解析JOIN和限定列名
+qualifiedName :: Parser String
+qualifiedName = do
+    first <- identifier
+    rest <- many (symbol "." *> identifier)
+    return (intercalate "." (first : rest))
+
 -- * 表达式层
 
 -- | 表达式入口，最低优先级。
@@ -114,7 +122,7 @@ atom =
     choice
         [ LitInt <$> integer
         , LitStr <$> stringLit
-        , Col <$> identifier
+        , Col <$> qualifiedName
         , between (symbol "(") (symbol ")") expr
         ]
 
@@ -123,7 +131,7 @@ atom =
 -- | 显式指定排序的语句
 orderItem :: Parser (String, SortDir)
 orderItem = do
-    col <- identifier
+    col <- qualifiedName
     dir <- sortDir
     return (col, dir)
 
@@ -140,28 +148,82 @@ assignment = do
     e <- expr
     return (col, e)
 
+-- | 从句语法
+
+-- | SQL 保留字，不能用作表别名
+reservedWords :: [String]
+reservedWords =
+    [ "select"
+    , "from"
+    , "where"
+    , "order"
+    , "by"
+    , "limit"
+    , "join"
+    , "on"
+    , "and"
+    , "or"
+    , "asc"
+    , "desc"
+    , "insert"
+    , "into"
+    , "values"
+    , "delete"
+    , "update"
+    , "set"
+    ]
+
+{- | 表别名。别名不能是保留字：否则 @FROM users WHERE age > 18@ 会把 @WHERE@
+当成 @users@ 的别名吃掉，后面真正的 WHERE 子句就再也解析不到。
+用 'try' 包住，一旦命中保留字就整体回溯，让 'optional' 正常返回 'Nothing'。
+-}
+aliasName :: Parser String
+aliasName = try $ do
+    name <- identifier
+    if map toLower name `elem` reservedWords then empty else return name
+
+tableRef :: Parser (Maybe String, String)
+tableRef = do
+    tbl <- identifier
+    mAlias <- optional aliasName
+    return (mAlias, tbl)
+
+joinClause :: Parser (Maybe String, String, Expr)
+joinClause = do
+    keyword "join"
+    (mAlias, tbl) <- tableRef
+    keyword "on"
+    cond <- expr
+    return (mAlias, tbl, cond)
+
+fromClause :: Parser FromClause
+fromClause = do
+    (mAlias, tbl) <- tableRef
+    joins <- many joinClause
+    return (foldl (\acc (a, t, c) -> FromJoin acc a t c) (FromTable mAlias tbl) joins)
+
 -- | 选择列表：@*@ 或逗号分隔的列名。
 selectList :: Parser [String]
 selectList =
     choice
         [ ["*"] <$ symbol "*"
-        , (:) <$> identifier <*> many (symbol "," *> identifier)
+        , (:) <$> qualifiedName <*> many (symbol "," *> qualifiedName)
         ]
 
 -- | @SELECT ... FROM ... [WHERE ...]@。
 selectQuery :: Parser Query
 selectQuery = do
-    _ <- keyword "select"
+    keyword "select"
     cols <- selectList
-    _ <- keyword "from"
+    keyword "from"
-    tableName <- identifier
+    fromC <- fromClause
     mWhere <- optional (keyword "where" *> expr)
     orderBy <- fromMaybe [] <$> optional orderByClause
     mLimit <- optional limitClause
     return
         Select
             { selectCols = cols
-            , selectTable = tableName
+            , selectFrom = fromC
             , selectWhere = mWhere
             , selectOrderBy = orderBy
             , selectLimit = mLimit

chusql-engine/src/ChuSQL/SQLSyntax/Sort.hs

@@ -1,7 +1,7 @@
-module ChuSQL.Eval.Sort where
+module ChuSQL.SQLSyntax.Sort where
 
 import ChuSQL.Model
-import ChuSQL.Syntax.Ast
+import ChuSQL.SQLSyntax.AST
 import Data.List (sortBy)
 
 -- | 反向排序

chusql-engine/test/Spec.hs

@@ -1,37 +1,50 @@
 module Main where
 
-import ChuSQL.Eval.Executor
 import ChuSQL.Model
-import ChuSQL.Syntax.Ast
+import ChuSQL.SQLSyntax.AST
-import ChuSQL.Syntax.Parser
+import ChuSQL.SQLSyntax.Executor
+import ChuSQL.SQLSyntax.Parser
 import Test.Hspec
 
+-- ============================================================
 -- Test database
-testDB :: Database
+-- ============================================================
-testDB = [("users", users)]
+users :: Table
-  where
+users =
-    users =
+    Table
-        Table
+        { tableName = "users"
-            { tableName = "users"
+        , tableCols = ["id", "name", "age"]
-            , tableCols = ["name", "age"]
+        , tableRows =
-            , tableRows =
+            [ [("id", VInt 1), ("name", VStr "Alice"), ("age", VInt 25)]
-                [ [("name", VStr "Alice"), ("age", VInt 25)]
+            , [("id", VInt 2), ("name", VStr "Bob"), ("age", VInt 17)]
-                , [("name", VStr "Bob"), ("age", VInt 17)]
+            , [("id", VInt 3), ("name", VStr "Carol"), ("age", VInt 30)]
-                , [("name", VStr "Carol"), ("age", VInt 30)]
+            ]
-                ]
+        }
-            }
 
+orders :: Table
+orders =
+    Table
+        { tableName = "orders"
+        , tableCols = ["id", "user_id", "product"]
+        , tableRows =
+            [ [("id", VInt 1), ("user_id", VInt 1), ("product", VStr "Book")]
+            , [("id", VInt 2), ("user_id", VInt 2), ("product", VStr "Pen")]
+            , [("id", VInt 3), ("user_id", VInt 1), ("product", VStr "Cup")]
+            ]
+        }
+
+testDB :: Database
+testDB = [("users", users), ("orders", orders)]
+
+-- ============================================================
+-- Main
+-- ============================================================
 main :: IO ()
 main = hspec $ do
     -- ============================================================
     -- AST
     -- ============================================================
     describe "ChuSQL.Syntax.Ast" $ do
-        it "shows a Select without WHERE" $ do
-            show (makeSelect ["name"] "users" Nothing)
-                `shouldBe` "Select {selectCols = [\"name\"], selectTable = \"users\", \
-                           \selectWhere = Nothing, selectOrderBy = [], selectLimit = Nothing}"
-
         it "equal Selects are equal" $ do
             makeSelect ["name"] "users" Nothing
                 `shouldBe` makeSelect ["name"] "users" Nothing
@@ -177,13 +190,56 @@ main = hspec $ do
                     )
 
         it "rejects trailing junk after a complete query" $ do
-            parseQuery "SELECT name FROM users extra"
+            parseQuery "SELECT name FROM users extra junk"
                 `shouldSatisfy` isLeft
 
         it "returns Left on missing table name" $ do
             parseQuery "SELECT name FROM"
                 `shouldSatisfy` isLeft
 
+    -- ============================================================
+    -- Parser: JOIN
+    -- ============================================================
+    describe "ChuSQL.Syntax.Parser (JOIN)" $ do
+        it "parses a simple JOIN without aliases" $ do
+            parseQuery "SELECT name FROM users JOIN orders ON id = user_id"
+                `shouldBe` Right
+                    ( Select
+                        { selectCols = ["name"]
+                        , selectFrom =
+                            FromJoin
+                                (FromTable Nothing "users")
+                                Nothing
+                                "orders"
+                                (Eq (Col "id") (Col "user_id"))
+                        , selectWhere = Nothing
+                        , selectOrderBy = []
+                        , selectLimit = Nothing
+                        }
+                    )
+
+        it "parses JOIN with aliases" $ do
+            parseQuery "SELECT u.name FROM users u JOIN orders o ON u.id = o.user_id"
+                `shouldBe` Right
+                    ( Select
+                        { selectCols = ["u.name"]
+                        , selectFrom =
+                            FromJoin
+                                (FromTable (Just "u") "users")
+                                (Just "o")
+                                "orders"
+                                (Eq (Col "u.id") (Col "o.user_id"))
+                        , selectWhere = Nothing
+                        , selectOrderBy = []
+                        , selectLimit = Nothing
+                        }
+                    )
+
+        it "parses qualified column names" $ do
+            case parseQuery "SELECT u.name, o.product FROM users u JOIN orders o ON u.id = o.user_id" of
+                Right q -> selectCols q `shouldBe` ["u.name", "o.product"]
+                Left err -> expectationFailure err
+
     -- ============================================================
     -- Engine: SELECT
     -- ============================================================
@@ -191,9 +247,9 @@ main = hspec $ do
         it "returns all rows without WHERE" $ do
             rowsOf (runQuery testDB (makeSelect ["*"] "users" Nothing))
                 `shouldBe` Right
-                    [ [("name", VStr "Alice"), ("age", VInt 25)]
+                    [ [("id", VInt 1), ("name", VStr "Alice"), ("age", VInt 25)]
-                    , [("name", VStr "Bob"), ("age", VInt 17)]
+                    , [("id", VInt 2), ("name", VStr "Bob"), ("age", VInt 17)]
-                    , [("name", VStr "Carol"), ("age", VInt 30)]
+                    , [("id", VInt 3), ("name", VStr "Carol"), ("age", VInt 30)]
                     ]
 
         it "filters with WHERE" $ do
@@ -261,7 +317,7 @@ main = hspec $ do
             case runQuery testDB (Insert "users" ["name", "age"] [LitStr "Dave", LitInt 22]) of
                 Left err -> expectationFailure err
                 Right (db', _) -> do
-                    rowsOf (runQuery db' (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery db' (makeSelect ["name", "age"] "users" Nothing))
                         `shouldBe` Right
                             [ [("name", VStr "Alice"), ("age", VInt 25)]
                             , [("name", VStr "Bob"), ("age", VInt 17)]
@@ -273,7 +329,7 @@ main = hspec $ do
             case runQuery testDB (Insert "users" ["name", "age"] [LitStr "Dave", LitInt 22]) of
                 Left err -> expectationFailure err
                 Right (_, _) -> do
-                    rowsOf (runQuery testDB (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery testDB (makeSelect ["name"] "users" Nothing))
                         `shouldSatisfy` \r -> case r of
                             Right rows -> length rows == 3
                             Left _ -> False
@@ -310,24 +366,24 @@ main = hspec $ do
             case runQuery testDB (Delete "users" (Just (Lt (Col "age") (LitInt 18)))) of
                 Left err -> expectationFailure err
                 Right (db', _) -> do
-                    rowsOf (runQuery db' (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery db' (makeSelect ["name"] "users" Nothing))
                         `shouldBe` Right
-                            [ [("name", VStr "Alice"), ("age", VInt 25)]
+                            [ [("name", VStr "Alice")]
-                            , [("name", VStr "Carol"), ("age", VInt 30)]
+                            , [("name", VStr "Carol")]
                             ]
 
         it "executes DELETE without WHERE, removing all rows" $ do
             case runQuery testDB (Delete "users" Nothing) of
                 Left err -> expectationFailure err
                 Right (db', _) -> do
-                    rowsOf (runQuery db' (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery db' (makeSelect ["name"] "users" Nothing))
                         `shouldBe` Right []
 
         it "does not modify the original database" $ do
             case runQuery testDB (Delete "users" Nothing) of
                 Left err -> expectationFailure err
                 Right (_, _) -> do
-                    rowsOf (runQuery testDB (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery testDB (makeSelect ["name"] "users" Nothing))
                         `shouldSatisfy` \r -> case r of
                             Right rows -> length rows == 3
                             Left _ -> False
@@ -380,7 +436,7 @@ main = hspec $ do
             case runQuery testDB (Update "users" [("age", LitInt 99)] (Just (Eq (Col "name") (LitStr "Alice")))) of
                 Left err -> expectationFailure err
                 Right (db', _) -> do
-                    rowsOf (runQuery db' (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery db' (makeSelect ["name", "age"] "users" Nothing))
                         `shouldBe` Right
                             [ [("name", VStr "Alice"), ("age", VInt 99)]
                             , [("name", VStr "Bob"), ("age", VInt 17)]
@@ -391,7 +447,7 @@ main = hspec $ do
             case runQuery testDB (Update "users" [("age", LitInt 0)] Nothing) of
                 Left err -> expectationFailure err
                 Right (db', _) -> do
-                    rowsOf (runQuery db' (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery db' (makeSelect ["name", "age"] "users" Nothing))
                         `shouldBe` Right
                             [ [("name", VStr "Alice"), ("age", VInt 0)]
                             , [("name", VStr "Bob"), ("age", VInt 0)]
@@ -402,7 +458,7 @@ main = hspec $ do
             case runQuery testDB (Update "users" [("age", LitInt 99), ("name", LitStr "X")] Nothing) of
                 Left err -> expectationFailure err
                 Right (db', _) -> do
-                    rowsOf (runQuery db' (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery db' (makeSelect ["name", "age"] "users" Nothing))
                         `shouldBe` Right
                             [ [("name", VStr "X"), ("age", VInt 99)]
                             , [("name", VStr "X"), ("age", VInt 99)]
@@ -413,7 +469,7 @@ main = hspec $ do
             case runQuery testDB (Update "users" [("age", LitInt 0)] Nothing) of
                 Left err -> expectationFailure err
                 Right (_, _) -> do
-                    rowsOf (runQuery testDB (makeSelect ["*"] "users" Nothing))
+                    rowsOf (runQuery testDB (makeSelect ["name"] "users" Nothing))
                         `shouldSatisfy` \r -> case r of
                             Right rows -> length rows == 3
                             Left _ -> False
@@ -465,7 +521,7 @@ main = hspec $ do
             let q =
                     Select
                         { selectCols = ["name"]
-                        , selectTable = "users"
+                        , selectFrom = FromTable Nothing "users"
                         , selectWhere = Nothing
                         , selectOrderBy = [("age", Asc)]
                         , selectLimit = Nothing
@@ -481,7 +537,7 @@ main = hspec $ do
             let q =
                     Select
                         { selectCols = ["name"]
-                        , selectTable = "users"
+                        , selectFrom = FromTable Nothing "users"
                         , selectWhere = Nothing
                         , selectOrderBy = [("age", Desc)]
                         , selectLimit = Nothing
@@ -497,7 +553,7 @@ main = hspec $ do
             let q =
                     Select
                         { selectCols = ["name"]
-                        , selectTable = "users"
+                        , selectFrom = FromTable Nothing "users"
                         , selectWhere = Nothing
                         , selectOrderBy = [("name", Asc)]
                         , selectLimit = Nothing
@@ -513,7 +569,7 @@ main = hspec $ do
             let q =
                     Select
                         { selectCols = ["name"]
-                        , selectTable = "users"
+                        , selectFrom = FromTable Nothing "users"
                         , selectWhere = Nothing
                         , selectOrderBy = [("nope", Asc)]
                         , selectLimit = Nothing
@@ -545,7 +601,7 @@ main = hspec $ do
             let q =
                     Select
                         { selectCols = ["name"]
-                        , selectTable = "users"
+                        , selectFrom = FromTable Nothing "users"
                         , selectWhere = Nothing
                         , selectOrderBy = []
                         , selectLimit = Just 2
@@ -560,7 +616,7 @@ main = hspec $ do
             let q =
                     Select
                         { selectCols = ["name"]
-                        , selectTable = "users"
+                        , selectFrom = FromTable Nothing "users"
                         , selectWhere = Nothing
                         , selectOrderBy = [("age", Desc)]
                         , selectLimit = Just 2
@@ -575,7 +631,7 @@ main = hspec $ do
             let q =
                     Select
                         { selectCols = ["name"]
-                        , selectTable = "users"
+                        , selectFrom = FromTable Nothing "users"
                         , selectWhere = Nothing
                         , selectOrderBy = []
                         , selectLimit = Just 10
@@ -591,7 +647,7 @@ main = hspec $ do
             let q =
                     Select
                         { selectCols = ["name"]
-                        , selectTable = "users"
+                        , selectFrom = FromTable Nothing "users"
                         , selectWhere = Nothing
                         , selectOrderBy = []
                         , selectLimit = Just 0
@@ -603,6 +659,76 @@ main = hspec $ do
             parseQuery "SELECT * FROM users LIMIT -1"
                 `shouldSatisfy` isLeft
 
+    -- ============================================================
+    -- Engine: JOIN
+    -- ============================================================
+    describe "ChuSQL.Eval.Engine (JOIN)" $ do
+        it "executes a two-table JOIN with aliases" $ do
+            rowsOf (parseQuery "SELECT u.name, o.product FROM users u JOIN orders o ON u.id = o.user_id" >>= runQuery testDB)
+                `shouldBe` Right
+                    [ [("u.name", VStr "Alice"), ("o.product", VStr "Book")]
+                    , [("u.name", VStr "Alice"), ("o.product", VStr "Cup")]
+                    , [("u.name", VStr "Bob"), ("o.product", VStr "Pen")]
+                    ]
+
+        it "executes JOIN with WHERE" $ do
+            rowsOf (parseQuery "SELECT u.name, o.product FROM users u JOIN orders o ON u.id = o.user_id WHERE u.age > 20" >>= runQuery testDB)
+                `shouldBe` Right
+                    [ [("u.name", VStr "Alice"), ("o.product", VStr "Book")]
+                    , [("u.name", VStr "Alice"), ("o.product", VStr "Cup")]
+                    ]
+
+        it "executes JOIN with ORDER BY" $ do
+            rowsOf (parseQuery "SELECT u.name, o.product FROM users u JOIN orders o ON u.id = o.user_id ORDER BY o.product DESC" >>= runQuery testDB)
+                `shouldBe` Right
+                    [ [("u.name", VStr "Bob"), ("o.product", VStr "Pen")]
+                    , [("u.name", VStr "Alice"), ("o.product", VStr "Cup")]
+                    , [("u.name", VStr "Alice"), ("o.product", VStr "Book")]
+                    ]
+
+        it "executes JOIN with LIMIT" $ do
+            rowsOf (parseQuery "SELECT u.name, o.product FROM users u JOIN orders o ON u.id = o.user_id LIMIT 2" >>= runQuery testDB)
+                `shouldBe` Right
+                    [ [("u.name", VStr "Alice"), ("o.product", VStr "Book")]
+                    , [("u.name", VStr "Alice"), ("o.product", VStr "Cup")]
+                    ]
+
+        it "returns Left when JOIN table not found" $ do
+            runQuery
+                testDB
+                ( Select
+                    { selectCols = ["u.name"]
+                    , selectFrom =
+                        FromJoin
+                            (FromTable (Just "u") "users")
+                            (Just "x")
+                            "nonexistent"
+                            (Eq (Col "u.id") (Col "x.id"))
+                    , selectWhere = Nothing
+                    , selectOrderBy = []
+                    , selectLimit = Nothing
+                    }
+                )
+                `shouldSatisfy` isLeft
+
+        it "returns Left when ON references an unknown column" $ do
+            runQuery
+                testDB
+                ( Select
+                    { selectCols = ["u.name"]
+                    , selectFrom =
+                        FromJoin
+                            (FromTable (Just "u") "users")
+                            (Just "o")
+                            "orders"
+                            (Eq (Col "u.nope") (Col "o.user_id"))
+                    , selectWhere = Nothing
+                    , selectOrderBy = []
+                    , selectLimit = Nothing
+                    }
+                )
+                `shouldSatisfy` isLeft
+
     -- ============================================================
     -- End-to-end
     -- ============================================================
@@ -652,7 +778,7 @@ main = hspec $ do
             case parseQuery "UPDATE users SET age = 99 WHERE name = 'Alice'" >>= runQuery testDB of
                 Left err -> expectationFailure err
                 Right (db', _) -> do
-                    rowsOf (parseQuery "SELECT * FROM users" >>= runQuery db')
+                    rowsOf (parseQuery "SELECT name, age FROM users" >>= runQuery db')
                         `shouldBe` Right
                             [ [("name", VStr "Alice"), ("age", VInt 99)]
                             , [("name", VStr "Bob"), ("age", VInt 17)]
@@ -666,18 +792,6 @@ main = hspec $ do
                     , [("name", VStr "Alice")]
                     ]
 
-        it "ORDER BY then INSERT then SELECT end-to-end" $ do
-            case parseQuery "INSERT INTO users (name, age) VALUES ('Dave', 99)" >>= runQuery testDB of
-                Left err -> expectationFailure err
-                Right (db', _) -> do
-                    rowsOf (parseQuery "SELECT name FROM users ORDER BY age DESC" >>= runQuery db')
-                        `shouldBe` Right
-                            [ [("name", VStr "Dave")]
-                            , [("name", VStr "Carol")]
-                            , [("name", VStr "Alice")]
-                            , [("name", VStr "Bob")]
-                            ]
-
         it "ORDER BY with LIMIT end-to-end" $ do
             rowsOf (parseQuery "SELECT name FROM users ORDER BY age DESC LIMIT 2" >>= runQuery testDB)
                 `shouldBe` Right
@@ -691,6 +805,14 @@ main = hspec $ do
                     [ [("name", VStr "Carol")]
                     ]
 
+        it "JOIN with WHERE and ORDER BY end-to-end" $ do
+            rowsOf (parseQuery "SELECT u.name, o.product FROM users u JOIN orders o ON u.id = o.user_id WHERE u.age > 15 ORDER BY o.product" >>= runQuery testDB)
+                `shouldBe` Right
+                    [ [("u.name", VStr "Alice"), ("o.product", VStr "Book")]
+                    , [("u.name", VStr "Alice"), ("o.product", VStr "Cup")]
+                    , [("u.name", VStr "Bob"), ("o.product", VStr "Pen")]
+                    ]
+
 -- ============================================================
 -- Helpers
 -- ============================================================

docs/INTRODUCTION.md

@@ -6,7 +6,7 @@
 
 ### 词法分析
 
-词法层与语法层同处 `chusql-engine/src/ChuSQL/Syntax/Parser.hs`，基于 **megaparsec** 实现。解析器类型为
+词法层与语法层同处 `chusql-engine/src/ChuSQL/SQLSyntax/Parser.hs`，基于 **megaparsec** 实现。解析器类型为
 
 ```haskell
 type Parser = Parsec Void String