📅  最后修改于: 2021-01-04 05:04:38             🧑  作者: Mango
SQLite UNION子句/运算符用于合并两个或多个SELECT语句的结果,而不返回任何重复的行。
要使用UNION,每个SELECT必须具有相同数量的选定列,相同数量的列表达式,相同数据类型,并且具有相同的顺序,但是它们的长度不必相同。
以下是UNION的基本语法。
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
UNION
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
在此,给定条件可以是根据您的要求的任何给定表达式。
考虑以下两个表,(a) 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
(b)另一个表格是部门—
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,让我们使用SELECT语句和UNION子句将这两个表连接起来,如下所示:
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 Engineering
3 Teddy Engineering
4 Mark Finance
5 David Engineering
6 Kim Finance
7 James Finance
UNION ALL运算符用于合并两个SELECT语句的结果,包括重复的行。
适用于UNION的相同规则也适用于UNION ALL运算符。
以下是UNION ALL的基本语法。
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
UNION ALL
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
在此,给定条件可以是根据您的要求的任何给定表达式。
现在,让我们在SELECT语句中将上述两个表连接如下:
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 Engineering
3 Teddy Engineering
4 Mark Finance
5 David Engineering
6 Kim Finance
7 James Finance
1 Paul IT Billing
2 Allen Engineering
3 Teddy Engineering
4 Mark Finance
5 David Engineering
6 Kim Finance
7 James Finance