实现基本关系代数运算，JOIN跨表查询

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

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+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

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+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

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+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"