Question

我正在尝试以下代码，并希望从table2或table3获取值，status1值可能是空白（空），也可能不是空白，并且不会得到结果。

它仅显示来自table2的结果，而不是来自table3。

如果我删除“a.status1 LIKE b.status1”，那么它可以正常工作。 status1在所有表中都是空的。

SELECT a.regno, 
a.status1, 
Coalesce(CASE 
          WHEN altervalue IS NOT NULL THEN altervalue 
          ELSE mastervalue 
        end, 0) AS value1 
FROM   table1 a 
LEFT JOIN (SELECT reg_no, total1+total2 AS altervalue 
          FROM   table2) AS i 
      ON `a`.`regno` LIKE `i`.`reg_no` 
LEFT JOIN (SELECT regno, total1+total2 AS mastervalue, 
                 status1 
          FROM   table3) AS b 
      ON a.status1 LIKE b.status1

Answer 1

您没有使用wildchar，因此您可以使用=代替like

SELECT 
  a.regno, 
  a.status1, 
  Coalesce(CASE 
          WHEN altervalue IS NOT NULL THEN altervalue 
          ELSE mastervalue 
        end, 0) AS value1 
FROM   table1 a 
LEFT JOIN (SELECT reg_no, total1+total2 AS altervalue 
          FROM   table2) AS i  ON `a`.`regno` =  `i`.`reg_no` 
LEFT JOIN (SELECT regno, total1+total2 AS mastervalue, 
                 status1 
          FROM   table3) AS b  ON a.status1 =  b.status1

或null安全比较

SELECT 
  a.regno, 
  a.status1, 
  Coalesce(CASE 
          WHEN altervalue IS NOT NULL THEN altervalue 
          ELSE mastervalue 
        end, 0) AS value1 
FROM   table1 a 
LEFT JOIN (SELECT reg_no, total1+total2 AS altervalue 
          FROM   table2) AS i  ON `a`.`regno` =  `i`.`reg_no` 
LEFT JOIN (SELECT regno, total1+total2 AS mastervalue, 
                 status1 
          FROM   table3) AS b  ON a.status1 <=>  b.status1

Answer 2

首先，简化您的查询：

SELECT t1.regno, t1.status1, 
       COALESCE(t2.total1 + t2.total2, t3.total1 + t3.total2, 0) as value1
FROM table1 t1 LEFT JOIN
     table2 t2
     ON t1.regno = t2.regno LEFT JOIN
     table3 t3
     ON t1.status1 = t3.status1 ;

我认为您的最终join缺少regno上的条件：

SELECT t1.regno, t1.status1, 
       COALESCE(t2.total1 + t2.total2, t3.total1 + t3.total2, 0) as value1
FROM table1 t1 LEFT JOIN
     table2 t2
     ON t1.regno = t2.regno LEFT JOIN
     table3 t3
     ON t3.regno = t1.regno AND t3.status1 = t1.status1 ;

我不确定你的真实逻辑（你没有样本数据/结果）。您可能希望以其他顺序使用COALESCE()：

       COALESCE(t3.total1 + t3.total2, t2.total1 + t2.total2, 0) as value1

选择mysql查询错误

2 个答案: