mysql - 如何加入 MySQL 查询

Question

我有 2 个要加入的表。

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是一个包含许多行的明细表，其中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，但它们都没有产生我想要的结果。

感谢您的帮助。

Answer 1

使用LEFT JOIN可能你错过了一些东西

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

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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

如果您的输出包含详细信息部分，那么您必须查看两行（因为它是有效结果），或者您需要通过选择 FIRST、MAX 等来聚合结果。

如果您本身不需要详细信息值，则替代方法是执行两个 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