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