如何从多个表中仅获取特定记录？

Question

我有表 ' table1 '、' table2 '、' table3 ' 和 ' table4 '。 “table1”有一个“account”列，因为我需要从“table2”或“table3”或“table4”中获取关于另一列“user_id”的记录。

我正在使用查询

SELECT * 
FROM table1 
LEFT OUTER JOIN table2 ON table1.user_id = table2.user_id
LEFT OUTER JOIN table2 ON table1.user_id = table2.user_id
LEFT OUTER JOIN table3 ON table1.user_id = table3.user_id

使用这个我得到了很多 NULL 值，因为 table1 的每条记录都只会在 'table2' 或 'table3' 或 'table4' 中有记录。

目前的输出就像 -

t1.user_id  | t1.post_id     | t2.token   | t3.token  | t4.token
 1          |      1         |   xyz      |   NULL    | NULL      
 1          |      1         |   NULL     |   abc     | NULL  
 1          |      1         |   NULL     |   NULL    | etc  

需要就像

 t1.user_id | t1.post_id     | token 
 1          |      1         |   xyz       
 1          |      1         |   abc  
 1          |      1         |   etc

t2、t3 和 t4 可以有 1 个以上的列可供选择。

Answer 1

使用COALESCE()函数会简单得多：

SELECT 
    t1.user_id, 
    t1.post_id, 
    COALESCE(t2.Token, t3.Token, t4.Token) AS Token
FROM table1 
LEFT OUTER JOIN table2 ON table1.user_id = table2.user_id
LEFT OUTER JOIN table2 ON table1.user_id = table2.user_id
LEFT OUTER JOIN table3 ON table1.user_id = table3.user_id

Answer 2

为此，您可以使用COALESCE()函数。

COALESCE() 函数返回列表中的第一个非空值。 例如

select COALESCE(NULL, NULL, NULL, 'abc', NULL, 'xyz');

上述查询的结果是abc 。

更新查询：

SELECT 
t1.user_id, 
t1.post_id, 
COALESCE(t2.Token, t3.Token, t4.Token) AS token 
FROM table1 
LEFT OUTER JOIN table2 ON table1.user_id = table2.user_id
LEFT OUTER JOIN table2 ON table1.user_id = table2.user_id
LEFT OUTER JOIN table3 ON table1.user_id = table3.user_id

Answer 3

SELECT * 
FROM table1 t1
INNER JOIN table2 t2 ON t1.user_id = t2.user_id
INNER JOIN table3 t3 ON t1.user_id = t3.user_id
INNER JOIN table4 t4 ON t1.user_id = t4.user_id

Answer 4

你可以这样做：

SELECT 
  t1.user_id,
  t1.post_id,
  MAX(t2.token) AS token2, 
  MAX(t3.token) AS token3, 
  MAX(t4.token) AS token4
FROM table1 AS t1
LEFT OUTER JOIN table2 AS t2 ON t1.user_id = t2.user_id
LEFT OUTER JOIN table3 AS t3 ON t1.user_id = t3.user_id
LEFT OUTER JOIN table4 AS t4 ON t1.user_id = t4.user_id
GROUP BY t1.user_id, t1.post_id