读书笔记--SQL必知必会14--组合查询
2016-12-27 23:36
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14.1 组合查询
复合查询(compound query)或并(union),SQL允许执行多个查询(多条SELECT语句),并将结果作为一个查询结果集返回。应用场景:
在一个查询中从不同的表返回结构数据
对一个表执行多个查询,按一个查询返回数据
多数情况下,任何具有多个WHERE子句的SELECT语句都可以作为一个组合查询。
14.2 创建组合查询
利用UNION操作符来组合多条SELECT语句。对于较复杂的过滤条件,或者从多个表中检索数据的情景,使用UNION可能会使处理更简单。
14.2.1 使用UNION
MariaDB [sqlbzbh]> SELECT cust_name, cust_contact, cust_email FROM Customers WHERE cust_state IN ('IL', 'IN', 'MI'); +---------------+--------------+-----------------------+ | cust_name | cust_contact | cust_email | +---------------+--------------+-----------------------+ | Village Toys | John Smith | sales@villagetoys.com | | Fun4All | Jim Jones | jjones@fun4all.com | | The Toy Store | Kim Howard | NULL | +---------------+--------------+-----------------------+ 3 rows in set (0.00 sec) MariaDB [sqlbzbh]> MariaDB [sqlbzbh]> SELECT cust_name, cust_contact,cust_email FROM Customers WHERE cust_name = 'Fun4All'; +-----------+--------------------+-----------------------+ | cust_name | cust_contact | cust_email | +-----------+--------------------+-----------------------+ | Fun4All | Jim Jones | jjones@fun4all.com | | Fun4All | Denise L. Stephens | dstephens@fun4all.com | +-----------+--------------------+-----------------------+ 2 rows in set (0.00 sec) MariaDB [sqlbzbh]> MariaDB [sqlbzbh]> SELECT cust_name, cust_contact, cust_email FROM Customers WHERE cust_state IN ('IL', 'IN', 'MI') -> UNION -> SELECT cust_name, cust_contact,cust_email FROM Customers WHERE cust_name = 'Fun4All'; +---------------+--------------------+-----------------------+ | cust_name | cust_contact | cust_email | +---------------+--------------------+-----------------------+ | Village Toys | John Smith | sales@villagetoys.com | | Fun4All | Jim Jones | jjones@fun4all.com | | The Toy Store | Kim Howard | NULL | | Fun4All | Denise L. Stephens | dstephens@fun4all.com | +---------------+--------------------+-----------------------+ 4 rows in set (0.00 sec) MariaDB [sqlbzbh]> MariaDB [sqlbzbh]> SELECT cust_name, cust_contact, cust_email FROM Customers WHERE cust_state IN ('IL', 'IN', 'MI') OR cust_name = 'Fun4All'; +---------------+--------------------+-----------------------+ | cust_name | cust_contact | cust_email | +---------------+--------------------+-----------------------+ | Village Toys | John Smith | sales@villagetoys.com | | Fun4All | Jim Jones | jjones@fun4all.com | | Fun4All | Denise L. Stephens | dstephens@fun4all.com | | The Toy Store | Kim Howard | NULL | +---------------+--------------------+-----------------------+ 4 rows in set (0.00 sec) MariaDB [sqlbzbh]>
14.2.2 UNION规则
必须由两条或两条以上的SELECT语句组成,之间用关键字UNION分隔
每个查询必须包含相同的列、表达式或聚集函数(次序可以不同)
列数据类型可以不同,但必须兼容
14.2.3 包含或取消重复的行
UNION默认自动从查询结果集中去除了重复的行。
如果想要返回所有匹配的行,可使用UNION ALL,不能使用WHERE。
MariaDB [sqlbzbh]> SELECT cust_name, cust_contact, cust_email FROM Customers WHERE cust_state IN ('IL', 'IN', 'MI') -> UNION ALL -> SELECT cust_name, cust_contact,cust_email FROM Customers WHERE cust_name = 'Fun4All'; +---------------+--------------------+-----------------------+ | cust_name | cust_contact | cust_email | +---------------+--------------------+-----------------------+ | Village Toys | John Smith | sales@villagetoys.com | | Fun4All | Jim Jones | jjones@fun4all.com | | The Toy Store | Kim Howard | NULL | | Fun4All | Jim Jones | jjones@fun4all.com | | Fun4All | Denise L. Stephens | dstephens@fun4all.com | +---------------+--------------------+-----------------------+ 5 rows in set (0.00 sec) MariaDB [sqlbzbh]>
14.2.4 对组合查询结果排序
使用UNION组合查询时,只能使用一条ORDER BY子句,而且必须位于最后一条SELECT语句之后。
ORDER BY子句排序所有SELECT语句返回的所有结果。
MariaDB [sqlbzbh]> SELECT cust_name, cust_contact, cust_email FROM Customers WHERE cust_state IN ('IL', 'IN', 'MI') -> UNION -> SELECT cust_name, cust_contact,cust_email FROM Customers WHERE cust_name = 'Fun4All' -> ORDER BY cust_name, cust_contact; +---------------+--------------------+-----------------------+ | cust_name | cust_contact | cust_email | +---------------+--------------------+-----------------------+ | Fun4All | Denise L. Stephens | dstephens@fun4all.com | | Fun4All | Jim Jones | jjones@fun4all.com | | The Toy Store | Kim Howard | NULL | | Village Toys | John Smith | sales@villagetoys.com | +---------------+--------------------+-----------------------+ 4 rows in set (0.00 sec) MariaDB [sqlbzbh]>
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