Question

我有三个执行COUNT的SELECT语句。认为将mysql结合起来会更有效率和更友善：

$query = $db->query("SELECT count(category) FROM table WHERE cid='290463' and category = 'x'"); 
         $tot_x = $db->result($query, 0);

$query = $db->query("SELECT count(category) FROM table WHERE cid='290463' and category = 'y'"); 
         $tot_y = $db->result($query, 0);

$query = $db->query("SELECT count(category) FROM table WHERE cid='290463' and category = 'z'"); 
         $tot_z = $db->result($query, 0);

一句话：

SELECT 
    SUM(category='x' AND cid='290463') as tot_x, 
    SUM(category='y' AND cid='290463') as tot_y, 
    SUM(category='z' AND cid='290463') as tot_z 
FROM table

问题是，新语句比分别运行原始语句慢。

任何人都能够了解为什么新声明会更慢以及建议如何更快地改进SELECT？

Answer 1

您的原始查询可能能够使用您的表上的索引（基于您的where子句），确保只读取表中相对较小比例的记录 - 而不是整个表，就像在您的表中一样统一查询。尝试将等效条件添加回where子句，如下所示：

SELECT 
    SUM(category='x') as tot_x, 
    SUM(category='y') as tot_y, 
    SUM(category='z') as tot_z 
FROM table
WHERE cid='290463' and category in ('x', 'y', 'z')

Answer 2

SELECT 
    SUM(IF(category='x',1,0))as tot_x, 
    SUM(IF(category='y',1,0))as tot_y, 
    SUM(IF(category='z',1,0))as tot_z
FROM table
WHERE cid='290463'
GROUP BY category

mysql在单个表中使用不同的where子句进行多次计数

2 个答案: