MySQL查询到从多个列

Question

指望我有SQL查询这样的：

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，...（包括随机顺序中的0,1,2）

我希望每个数字的总数为：0 => 2,1 => 3,2 => 1，无PHP代码

Answer 1

你可以简单地包裹在一个子查询现有查询，并进行计数的出现外查询;

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; 

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