mysql - 从子查询中获取总行数？

Question

我有一个像这样的查询

SELECT row1, row2
FROM (
    SELECT b.row21, COUNT(b.row21) AS hits
    FROM table2 AS b
    WHERE b.row22 = 'foo' OR b.row22 = 'bar'
    GROUP BY b.row21
    HAVING hits = 2
) AS b
INNER JOIN table1 AS a ON (b.row21 = a.row1)
WHERE row2 = 123
LIMIT 10

现在很明显，结果首先受到附加限制WHERE，最后受到限制LIMIT。

那么如何在不需要单独执行的情况下检索子查询返回的行数呢？

Answer 1

您可以在 MySQL 中使用变量执行此操作：

SELECT row1, row2, @rn as numrows
FROM (
    SELECT b.row21, COUNT(b.row21) AS hits, @rn := @rn + 1
    FROM table2 b cross join
         (select @rn := 0) const
    WHERE b.row22 = 'foo' OR b.row22 = 'bar'
    GROUP BY b.row21
    HAVING hits = 2
) AS b
INNER JOIN table1 AS a ON (b.row21 = a.row1)
WHERE row2 = 123
LIMIT 10;

您冒着 MySQL可能以非常不同的方式重写查询的风险，这可能会避免生成某些行。一些 SQL 编译器足够聪明地做到这一点。我不认为 MySQL 有那么聪明。