实现基本关系代数运算，JOIN跨表查询

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