mysql select 查询错误

Question

我正在尝试以下代码，并希望从 table2 或 table3 中获取值，status1 值可能为空，也可能不为空，并且不会获得结果。

它仅显示来自表 2 的结果，而不显示来自表 3 的结果。

如果我删除“a.status1 LIKE b.status1”那么它工作正常。所有表中的 status1 均为空。

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

您没有使用通配符，因此可以使用 = 而不是 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 

或空安全比较

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