MySQL查询从多个列计数

Question

我有这样的SQL查询：

SELECT
   CONCAT(t1.a, t2.a, t2.b, t3.a, t4.a, t4.b) AS val
FROM
   t1 
   LEFT JOIN t2 ON ...
   LEFT JOIN t3 ON ...
   LEFT JOIN t4 ON ...

val输出为：0,1,0,1,2,1或1,2,0,1,0,2，...（以随机顺序包括0,1,2）

我想对每个数字进行总计，如：0 => 2、1 => 3、2 => 1，不带PHP代码

Answer 1

您可以简单地将现有查询包装在子查询中，然后进行外部查询以计算发生的次数。

SELECT 
  LENGTH(val) - LENGTH(REPLACE(val, '0', '')) zeros,
  LENGTH(val) - LENGTH(REPLACE(val, '1', '')) ones,
  LENGTH(val) - LENGTH(REPLACE(val, '2', '')) twos
FROM (
  SELECT
    CONCAT(t1.a, t2.a, t2.b, t3.a, t4.a, t4.b) AS val
  FROM
    t1 
    LEFT JOIN t2 ON ...
    LEFT JOIN t3 ON ...
    LEFT JOIN t4 ON ...
);

Answer 2

您需要为此使用UNION ALL 。

SELECT tbl.col1, COUNT(tbl.col1)
FROM (
  SELECT t1.a as col1
    FROM t1
  UNION ALL
  SELECT t2.a as col1
    FROM t2
  UNION ALL
  SELECT t3.a as col1
    FROM t3
   ...
) tbl
GROUP BY tbl.col1;

SQLFIDDLE演示