Union和Union All的区别

使用mysql数据库中 mysql库中的 表 help_category


1>UNION 自动去除 重复数据，UNION  ALL 不会

SELECT * FROM help_category t WHERE t.help_category_id<=7
UNION
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10 ;

结果：
   

< help_category_id name parent_category_id 1 Geographic 0 2 Polygon properties 33 3 WKT 33 4 Numeric Functions 37 5 Plugins 34 6 MBR 33 7 Control flow functions 37 8 Transactions 34 9 Help Metadata 34 10 Account Management 34

 
SELECT * FROM help_category t WHERE t.help_category_id<=7
UNION  ALL
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10 ;

结果：

< help_category_id name parent_category_id url 1 Geographic 0   2 Polygon properties 33   3 WKT 33   4 Numeric Functions 37   5 Plugins 34   6 MBR 33   7 Control flow functions 37   5 Plugins 34   6 MBR 33   7 Control flow functions 37   8 Transactions 34   9 Help Metadata 34   10 Account Management 34

2>顺序问题
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10  
UNION
SELECT * FROM help_category t WHERE t.help_category_id<=7;

< help_category_id name parent_category_id url 5 Plugins 34   6 MBR 33   7 Control flow functions 37   8 Transactions 34   9 Help Metadata 34   10 Account Management 34   1 Geographic 0   2 Polygon properties 33   3 WKT 33   4 Numeric Functions 37

调换顺序后，结果无序

SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10  
UNION ALL
SELECT * FROM help_category t WHERE t.help_category_id<=7;

< help_category_id name parent_category_id url 5 Plugins 34   6 MBR 33   7 Control flow functions 37   8 Transactions 34   9 Help Metadata 34   10 Account Management 34   1 Geographic 0   2 Polygon properties 33   3 WKT 33   4 Numeric Functions 37   5 Plugins 34   6 MBR 33   7 Control flow functions 37

同样 ，调换顺序后结果无序

所以为了保证数据的有序，需要在 最后那个sql 中加上order by

SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10  
UNION ALL
SELECT * FROM help_category t WHERE t.help_category_id<=7 ORDER BY help_category_id;

< help_category_id name parent_category_id url 1 Geographic 0   2 Polygon properties 33   3 WKT 33   4 Numeric Functions 37   5 Plugins 34   5 Plugins 34   6 MBR 33   6 MBR 33   7 Control flow functions 37   7 Control flow functions 37   8 Transactions 34   9 Help Metadata 34   10 Account Management 34

SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10  
UNION
SELECT * FROM help_category t WHERE t.help_category_id<=7 ORDER BY help_category_id;

< help_category_id name parent_category_id url 1 Geographic 0   2 Polygon properties 33   3 WKT 33   4 Numeric Functions 37   5 Plugins 34   6 MBR 33   7 Control flow functions 37   8 Transactions 34   9 Help Metadata 34   10 Account Management 34

Union，对两个结果集进行并集操作，不包括重复行

Union All，对两个结果集进行并集操作，包括重复行

最后把所有的sql脚本保留一下

SELECT * FROM help_category t WHERE t.help_category_id<=7 
UNION 
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10 ;

SELECT * FROM help_category t WHERE t.help_category_id<=7 
UNION ALL
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10 ;

--=================================================================================
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10  
UNION 
SELECT * FROM help_category t WHERE t.help_category_id<=7 ORDER BY help_category_id;

SELECT * FROM help_category t WHERE t.help_category_id<=7 
UNION 
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10 ORDER BY help_category_id;

--=================================================================================
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10  
UNION ALL
SELECT * FROM help_category t WHERE t.help_category_id<=7 ORDER BY help_category_id;

SELECT * FROM help_category t WHERE t.help_category_id<=7 
UNION ALL
SELECT * FROM help_category t WHERE t.help_category_id>=5 AND t.help_category_id<=10 ORDER BY help_category_id;