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。
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]
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
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
---------- -------------------- ----------
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;
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
---------- -------------------- ----------
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]
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;
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
---------- -------------------- ----------
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
