实现基本关系代数运算，JOIN跨表查询

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

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+module ChuSQL.SQLSyntax.Sort where
+
+import ChuSQL.Model
+import ChuSQL.SQLSyntax.AST
+import Data.List (sortBy)
+
+-- | 反向排序
+flipOrdering :: Ordering -> Ordering
+flipOrdering LT = GT
+flipOrdering GT = LT
+flipOrdering EQ = EQ
+
+-- | 定义值的比较规则
+compareValue :: Value -> Value -> Ordering
+compareValue (VInt a) (VInt b) = compare a b
+compareValue (VStr a) (VStr b) = compare a b
+compareValue (VBool a) (VBool b) = compare a b
+compareValue _ _ = EQ
+
+-- | 定义行的比较规则
+compareRows :: [(String, SortDir)] -> Row -> Row -> Ordering
+compareRows [] _ _ = EQ
+compareRows ((col, dir) : rest) r1 r2 =
+    case (lookup col r1, lookup col r2) of
+        (Just v1, Just v2) ->
+            case compareValue v1 v2 of
+                EQ -> compareRows rest r1 r2
+                o ->
+                    if dir == Desc
+                        then flipOrdering o
+                        else o
+        _ -> compareRows rest r1 r2
+
+-- | 将表的每行排序
+sortRows :: [(String, SortDir)] -> [Row] -> [Row]
+sortRows [] rows = rows
+sortRows spec rows = sortBy (compareRows spec) rows