Question

假设我有两张桌子：

 table1
   table1_id
   table1_name

 table2
   table2_id
   table2_name
   table2_description
   table1_id

我这样加入：

 SELECT * FROM table1 LEFT JOIN table2 ON table1.table1_id = table2.table1_id

如果table2中没有匹配项，我如何让table1_id返回其默认值而不是NULL？

答案：由于table1_id存在于两个表中，我需要使用别名。

 SELECT *, table1.table1_id AS tid, table2.table1_id AS t2id FROM table1 LEFT JOIN table2 ON table1.table1_id = table2.table1_id

Answer 1

如果我理解正确，你会引入默认行，然后使用select中的逻辑：

SELECT t1.*,
       COALESCE(t2.table2_name, t2def.table2_name) as table2_name,
       . . .
FROM table1 t1 LEFT JOIN
     table2 t2
     ON t1.table1_id = t2.table1_id LEFT JOIN
     table2 t2def
     ON t2def.table1_id = $defaultid;

Answer 2

SELECT table1.*
  table2.table2_id,
  table2.table2_name,
  table2.table2_description,
  ifnull(table2.table1_id, table1.table1_id) t2id
FROM table1 LEFT JOIN table2 
ON table1.table1_id = table2.table1_id

同样，没有必要检查table2.table1_id是否为null，因为它是连接列...始终为NULL或与table1.table1_id完全相同

MySQL JOIN - 不要NULL值

2 个答案: