Question

我发现this tutorial关于SQL EXISTS运算符。我试图了解我们为什么需要它。始终可以用另一个表达式替换“ EXISTS”（如下所示）

例如，此SQL：

SELECT count(SupplierName)
FROM Suppliers
WHERE EXISTS (SELECT * FROM Products WHERE SupplierId = Suppliers.supplierId AND Price < 20);

可以替换为此SQL：

SELECT count(SupplierName)
FROM Suppliers
WHERE  (SELECT count(*) FROM Products WHERE SupplierId = Suppliers.supplierId AND Price < 20) > 0;

我对其进行了测试，并得到了相同的结果。

在任何情况下我们都必须使用“ EXISTS”吗？

Answer 1

在您的示例中，EXISTS效率更高。子查询需要读取所有个匹配行以进行计数。

根据合同，

EXISTS可以（并且确实！）停在匹配的第一行。

您可能会说SQL引擎可以识别这种情况。但是，对于更复杂的查询，这将非常非常困难。

与您的观点相反，我发现EXISTS比IN或具有聚合功能的相关子查询还要有用。这是表达此逻辑的首选方法。

Answer 2

在性能方面，EXISTS更好，因为它只需要检索单行而不是枚举所有行。

http://sqlblog.com/blogs/andrew_kelly/archive/2007/12/15/exists-vs-count-the-battle-never-ends.aspx

Answer 3

在某些情况下，Exist可能会有更好的性能。例如，要执行以下查询，引擎必须计算子查询中的所有行：

SELECT count(SupplierName)
FROM Suppliers
WHERE  (SELECT count(*) FROM Products WHERE SupplierId = Suppliers.supplierId AND Price < 20) > 0;

在这种情况下，如果找到第一个产品，它将产生收益；

SELECT count(SupplierName)
FROM Suppliers
WHERE EXISTS (SELECT * FROM Products WHERE SupplierId = Suppliers.supplierId AND Price < 20);

Answer 4

在大多数情况下，EXISTS()更具表现力。例如，尝试将以下片段重编为NOT COUNT(*) > 0版本。哪一个更具可读性？

        --
        -- All the possible moves for the current board.
        -- ,which is := all the (empty spots * all the numbers  1-9)
        -- for which the same number does not occur yet in the same {row/col/box}.
        --
CREATE VIEW valid_moves AS (
        SELECT DISTINCT su.iii AS iii
                , su.yyy AS yyy
                , su.xxx AS xxx
                , su.box AS box
                , su.y3 AS y3
                , su.x3 AS x3
                , su.z3 AS z3
                , nn.val AS val
        FROM v_sudoku su            -- current board, including filled-in numbers
        CROSS JOIN all_numbers nn   -- Cartesian product here ...
        WHERE su.val IS NULL        -- empty spots
        AND NOT EXISTS (SELECT * FROM v_sudoku ny
                WHERE ny.yyy = su.yyy AND ny.val = nn.val
                )
        AND NOT EXISTS (SELECT * FROM v_sudoku nx
                WHERE nx.xxx = su.xxx AND nx.val = nn.val
                )
        AND NOT EXISTS (SELECT * FROM v_sudoku nz
                WHERE nz.box = su.box AND nz.val = nn.val
                )
        );

为什么在SQL中需要“ WHERE EXISTS”运算符？

4 个答案: