SQLite的 UNION 子句/運算符用于合并兩個或多個 SELECT 語句的結(jié)果,不返回任何重復(fù)的行。
為了使用 UNION,每個 SELECT 被選擇的列數(shù)必須是相同的,相同數(shù)目的列表達式,相同的數(shù)據(jù)類型,并確保它們有相同的順序,但它們不必具有相同的長度。
UNION 的基本語法如下:
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
UNION
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
這里給定的條件根據(jù)需要可以是任何表達式。
假設(shè)有下面兩個表,(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)另一個表是 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
現(xiàn)在,讓我們使用 SELECT 語句及 UNION 子句來連接兩個表,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;
這將產(chǎn)生以下結(jié)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 運算符用于結(jié)合兩個 SELECT 語句的結(jié)果,包括重復(fù)行。
適用于 UNION 的規(guī)則同樣適用于 UNION ALL 運算符。
UNION ALL 的基本語法如下:
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
UNION ALL
SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]
這里給定的條件根據(jù)需要可以是任何表達式。
現(xiàn)在,讓我們使用 SELECT 語句及 UNION ALL 子句來連接兩個表,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;
這將產(chǎn)生以下結(jié)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