MySQL查詢到從多個列

Question

指望我有SQL查詢這樣的：

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輸出爲：0,1,0,1,2,1或1,2,0,1,0， 2，...（包括隨機順序中的0,1,2）

我希望每個數字的總數爲：0 => 2,1 => 3,2 => 1，無PHP代碼

Answer 1

你可以簡單地包裹在一個子查詢現有查詢，並進行計數的出現外查詢;

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

您需要爲此使用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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