Question

我有一张tbl_customers表。（像许多人一样）

我有第二张桌子：tbl_customers_tags

这张表只是让我为单个客户记录存储无限的关键字/标签。这是列结构：

`tbl_customers_tags`

+-------------+---------------------+------+-----+---------+----------------+
| Field       | Type                | Null | Key | Default | Extra          |
+-------------+---------------------+------+-----+---------+----------------+
| ID          | int(11) unsigned    | NO   | PRI | NULL    | auto_increment |
| customerid  | int(11) unsigned    | NO   | MUL | NULL    |                |
| tagid       | mediumint(5)unsigned| NO   |     | NULL    |                |

所以，我现在需要通过查找具有某些tagid的任何客户端记录来提取一些报告。我正在重复。这是我当前的查询语句：

SELECT  c.firstname, c.lastname, c.datecreated  
FROM `tbl_customers` c 
LEFT JOIN `tbl_customers_tags` ctags ON c.customerid = ctags.customerid 
WHERE ctags.tagid IN(2,3,15)

当单个customerid与多个tagid记录相关联时，是否有办法不返回重复项？

Answer 1

SELECT  distinct c.firstname, c.lastname, c.datecreated  
FROM `tbl_customers` c 
LEFT JOIN `tbl_customers_tags` ctags ON c.customerid = ctags.customerid 
WHERE ctags.tagid IN(2,3,15)

Answer 2

如果您真的不关心重复项，可以使用DISTINCT关键字。此外，您不需要执行LEFT OUTER JOIN，因为您有WHERE子句依赖于另一个表。像这样：

SELECT  DISTINCT c.firstname, c.lastname, c.datecreated  
FROM `tbl_customers` c 
JOIN `tbl_customers_tags` ctags 
ON c.customerid = ctags.customerid 
WHERE ctags.tagid IN (2,3,15)

Answer 3

试试这个

SELECT  c.firstname, c.lastname, c.datecreated  
FROM 'tbl_customers' c 
where exists
( select 'x' from tbl_customers_tags where customerid = c.customerid and
tagid IN(2,3,15)
)

如果您只对标签表中有记录的客户感兴趣，为什么不进行完全加入而不是左转？

MySQL - 一对多连接语句返回重复项

3 个答案: