Question

我有2张桌子要加入。

Table 1是一个主表，包含一组唯一的行。

ID DESCRIPTION
01 Alpha
02 Gamma
03 Delta
04 Beta

Table 2是一个详细信息表，其中包含许多行，只有一些行与主表匹配。

ID   INFO MASTER_ID
0001 abc  02
0002 def  02
0004 xyz  01
0005 mno  04

我想加入表格以产生以下结果：

Alpha "has detail"
Gamma "has detail"
Delta "does not have detail"
Beta "has detail"

而不是：

Alpha "has detail"
Gamma "has detail"
Gamma "has detail"
Delta "does not have detail"
Beta "has detail"

后面的输出返回Gamma两次，我只希望它返回一次。

我尝试了LEFT JOIN，RIGHT JOIN，INNER JOIN，但没有一个能产生我想要的结果。

感谢您的帮助。

Answer 1

使用LEFT JOIN可能你错过了什么

SELECT  CONCAT(a. description, IF(b.master_ID IS NOT NULL, ' has detail',' does not have detail'))
FROM    table1 a
        LEFT JOIN (SELECT DISTINCT master_ID FROM table2) b
            ON a.ID = b.master_ID

Answer 2

也许是这样的：

SELECT
    Table1.DESCRIPTION,
    (
        CASE 
            WHEN EXISTS(SELECT NULL FROM Table2 WHERE Table2.MASTER_ID=Table1.ID)
            THEN 'has detail'
            ELSE 'does not have detail'
        END
    ) AS has
FROM
    Table1

Answer 3

只是一种不同的方法：

SELECT
  table1.description,
  IF((EXISTS(SELECT master_id from table2  where table2.master_id = table1.id)),
    'has detail',
    'does not have detail') As Status
FROM table1

Answer 4

SELECT a.Description, IF(b.Info, "has detail", "does not have detail") as Status
FROM Table_1 a
LEFT OUTER JOIN Table_2 b
ON a.MASTER_ID = b.MASTER_ID
GROUP BY 1,2;

Answer 5

如果你的输出包含细节部分，那么你必须要么看到两行（因为它是有效的结果），要么你需要通过选择FIRST，MAX等来聚合结果。

如果你自己不需要细节值，那么替代方法是做两个SELECT并使用UNION：

    SELECT DISTINCT Table1.Description, "does not have detail" as Comment
    FROM Table1
    LEFT JOIN Table2 ON (Table1.ID = MASTER_ID)
    WHERE Table2.id IS NULL

    UNION 

    SELECT DISTINCT Table1.Description, "has detail" as Comment
    FROM Table1
    LEFT JOIN Table2 ON (Table1.ID = MASTER_ID)
    WHERE Table2.id IS NOT NULL

Answer 6

您也可以使用groupby

来实现此目的

SELECT a.Description, IF(count(b.id) > 0, "has detail", "does not have detail") as Status
FROM Table1 a
LEFT OUTER JOIN Table2 b
ON a.iD = b.MASTER_ID
GROUP BY a.Description ORDER BY a.Description ASC

如何加入MySQL查询

6 个答案: