MySQL查詢從多個列計數

Question

我有這樣的SQL查詢：

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輸出為：0,1,0,1,2,1或1,2,0,1,0,2，...（以隨機順序包括0,1,2）

我想對每個數字進行總計，如：0 => 2、1 => 3、2 => 1，不帶PHP代碼

Answer 1

您可以簡單地將現有查詢包裝在子查詢中，然后進行外部查詢以計算發生的次數。

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

您需要為此使用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