SQLite UNIONS 子句

SQLite的UNION子句/运算符用于合并两个或多个SELECT语句的结果,不返回任何重复的行。

直接使用UNION,每个SELECT选择的列数必须具有相同的,相同数目的列表达式相同的数据类型,并让它们在相同的顺序,但它们不必具有相同的长度。

语法

UNION的基本语法如下:

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

UNION

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

这里给定的条件可以是任何表达式,根据需要。

例子:

考虑以下两个表:COMPANY表如下:

sqlite> select * from COMPANY;
ID          NAME                  AGE         ADDRESS     SALARY
----------  --------------------  ----------  ----------  ----------
1           Paul                  32          California  20000.0
2           Allen                 25          Texas       15000.0
3           Teddy                 23          Norway      20000.0
4           Mark                  25          Rich-Mond   65000.0
5           David                 27          Texas       85000.0
6           Kim                   22          South-Hall  45000.0
7           James                 24          Houston     10000.0

另一表是DEPARTMENT如下:

ID          DEPT                  EMP_ID
----------  --------------------  ----------
1           IT Billing            1
2           Engineering           2
3           Finance               7
4           Engineering           3
5           Finance               4
6           Engineering           5
7           Finance               6

现在,让我们加入这两个表使用SELECT语句一起UNION子句如下:

sqlite> SELECT EMP_ID, NAME, DEPT FROM COMPANY INNER JOIN DEPARTMENT
        ON COMPANY.ID = DEPARTMENT.EMP_ID
   UNION
     SELECT EMP_ID, NAME, DEPT FROM COMPANY LEFT OUTER JOIN DEPARTMENT
        ON COMPANY.ID = DEPARTMENT.EMP_ID;

这将产生以下结果:

EMP_ID      NAME                  DEPT
----------  --------------------  ----------
1           Paul                  IT Billing
2           Allen                 Engineerin
3           Teddy                 Engineerin
4           Mark                  Finance
5           David                 Engineerin
6           Kim                   Finance
7           James                 Finance

UNION ALL 子句:

UNION ALL运算符是用来结合两个SELECT语句,包括重复行的结果。

UNION 适用同样的规则适用于UNION 所有操作符。

语法

UNION ALL的基本语法如下:

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

UNION ALL

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

这里给定的条件可以是任何表达式,根据需要。

例子:

现在,让我们连接上述两个表中的SELECT语句如下:

sqlite> SELECT EMP_ID, NAME, DEPT FROM COMPANY INNER JOIN DEPARTMENT
        ON COMPANY.ID = DEPARTMENT.EMP_ID
   UNION ALL
     SELECT EMP_ID, NAME, DEPT FROM COMPANY LEFT OUTER JOIN DEPARTMENT
        ON COMPANY.ID = DEPARTMENT.EMP_ID;

这将产生以下结果:

EMP_ID      NAME                  DEPT
----------  --------------------  ----------
1           Paul                  IT Billing
2           Allen                 Engineerin
3           Teddy                 Engineerin
4           Mark                  Finance
5           David                 Engineerin
6           Kim                   Finance
7           James                 Finance
1           Paul                  IT Billing
2           Allen                 Engineerin
3           Teddy                 Engineerin
4           Mark                  Finance
5           David                 Engineerin
6           Kim                   Finance
7           James                 Finance

        原文作者:SQLite教程
        原文地址: https://www.yiibai.com/sqlite/sqlite_unions_clause.html
        本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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