Question

$SQL = "SELECT pc.dist_id, pc.route, pc.types,
        COUNT( pc.types = '$type' ) total,
        COUNT( pc.types = '$type' AND pc.options = '1') callbacks

        FROM per_call pc
        WHERE pc.types = '$type'
        GROUP BY pc.dist_id, pc.route, pc.types
        ORDER BY pc.dist_id, pc.route";

简而言之，我想我是在问COUNT如何处理（）中的多个列。

Answer 1

Count也可以使用：

SELECT pc.dist_id, pc.route, pc.types,
COUNT( CASE WHEN pc.types = '$type' THEN 1 END) total,
COUNT( CASE WHEN pc.types = '$type' AND pc.options = '1' THEN 1 END) callbacks

FROM per_call pc
WHERE pc.types = '$type'
GROUP BY pc.dist_id, pc.route, pc.types
ORDER BY pc.dist_id, pc.route

注意：它有效，因为COUNT返回非NULL值的数量，而CASE具有默认ELSE NULL ...

Answer 2

另一个略短的变体（性能与CASE语句大致相同）：

SELECT pc.dist_id, pc.route, pc.types
      ,COUNT(NULLIF(pc.types = '$type', FALSE) AS total
      ,COUNT(NULLIF(pc.types = '$type' AND pc.options = '1', FALSE) AS callbacks
FROM   per_call pc
WHERE  pc.types = '$type'
GROUP  BY pc.dist_id, pc.route, pc.types
ORDER  BY pc.dist_id, pc.route;

背后的主要是： COUNT计算所有非空值表达式产生一个布尔值，可以是TRUE，FALSE或NULL 您只想计算TRUE案例因此，将FALSE转换为NULL，一切都很常规。

或者，甚至更短，但不是更快：

SELECT pc.dist_id, pc.route, pc.types
      ,sum((pc.types = '$type')::int) AS total
      ,sum((pc.types = '$type' AND pc.options = '1')::int) AS callbacks
FROM   per_call pc
WHERE  pc.types = '$type'
GROUP  BY pc.dist_id, pc.route, pc.types
ORDER  BY pc.dist_id, pc.route;

将boolean强制转换为integer，将TRUE转换为1，将FALSE转换为0.因此，sum()有效。

由于您的pc.types = '$type'条件为WHERE，因此您可以简化为：

SELECT pc.dist_id, pc.route, pc.types
      ,COUNT(*) AS total  -- slightly faster, too
      ,COUNT(NULLIF(pc.options = '1', FALSE) AS callbacks
FROM   per_call pc
WHERE  pc.types = '$type'
GROUP  BY pc.dist_id, pc.route, pc.types
ORDER  BY pc.dist_id, pc.route;

为什么这两个计数返回相同的数字？

2 个答案: