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。

为了使用 UNION,每个 SELECT 被选择的列数必须是相同的,相同数目的列表达式,相同的数据类型,并确保它们有相同的顺序,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]

这里给定的条件根据需要可以是任何表达式。

实例

假设有下面两个表,(1)COMPANY 表如下所示:

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

(2)另一个表是 DEPARTMENT,如下所示:

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 语句及 UNION 子句来连接两个表,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 Engineerin
3 Teddy Engineerin
4 Mark Finance
5 David Engineerin
6 Kim Finance
7 James Finance

UNION ALL 子句

UNION ALL 运算符用于结合两个 SELECT 语句的结果,包括重复行。

适用于 UNION 的规则同样适用于 UNION ALL 运算符。

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]

这里给定的条件根据需要可以是任何表达式。

实例

现在,让我们使用 SELECT 语句及 UNION ALL 子句来连接两个表,如下所示:

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