SQL UNION子句/运算符用于组合两个或多个SELECT语句的结果,而不返回任何重复的行.
要使用此UNION子句,每个SELECT语句必须具有
选择的列数相同
相同数量的列表达式
相同的数据类型和
按相同顺序排列
但他们需要不必长度相同.
UNION 子句的基本语法如下: ;
SELECT column1 [,column2] FROM table1 [,table2] [WHERE condition] UNION SELECT column1 [,column2] FROM table1 [,table2] [WHERE condition]
这里,给定条件可以是基于您的要求的任何给定表达式.
考虑以下两个表./p>
表1 : CUSTOMERS表如下.
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
表2 : ORDERS表如下.
+-----+---------------------+-------------+--------+ |OID | DATE | CUSTOMER_ID | AMOUNT | +-----+---------------------+-------------+--------+ | 102 | 2009-10-08 00:00:00 | 3 | 3000 | | 100 | 2009-10-08 00:00:00 | 3 | 1500 | | 101 | 2009-11-20 00:00:00 | 2 | 1560 | | 103 | 2008-05-20 00:00:00 | 4 | 2060 | +-----+---------------------+-------------+--------+
现在,让我们在SELECT语句中加入这两个表,如下所示 :
SQL> SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS LEFT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID UNION SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS RIGHT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
这会产生以下结果 :
+------+----------+--------+---------------------+ | ID | NAME | AMOUNT | DATE | +------+----------+--------+---------------------+ | 1 | Ramesh | NULL | NULL | | 2 | Khilan | 1560 | 2009-11-20 00:00:00 | | 3 | kaushik | 3000 | 2009-10-08 00:00:00 | | 3 | kaushik | 1500 | 2009-10-08 00:00:00 | | 4 | Chaitali | 2060 | 2008-05-20 00:00:00 | | 5 | Hardik | NULL | NULL | | 6 | Komal | NULL | NULL | | 7 | Muffy | NULL | NULL | +------+----------+--------+---------------------+
UNION ALL运算符用于组合两个SELECT的结果包含重复行的语句.
适用于UNION子句的相同规则将应用于UNION ALL运算符.
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,给定条件可以是基于您的要求的任何给定表达式.
考虑以下两个表,
表1 : CUSTOMERS表如下.
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
表2 : ORDERS表如下.
+-----+---------------------+-------------+--------+ |OID | DATE | CUSTOMER_ID | AMOUNT | +-----+---------------------+-------------+--------+ | 102 | 2009-10-08 00:00:00 | 3 | 3000 | | 100 | 2009-10-08 00:00:00 | 3 | 1500 | | 101 | 2009-11-20 00:00:00 | 2 | 1560 | | 103 | 2008-05-20 00:00:00 | 4 | 2060 | +-----+---------------------+-------------+--------+
现在,让我们在SELECT语句中加入这两个表,如下所示 :
SQL> SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS LEFT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID UNION ALL SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS RIGHT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;
这会产生以下结果 :
+------+----------+--------+---------------------+ | ID | NAME | AMOUNT | DATE | +------+----------+--------+---------------------+ | 1 | Ramesh | NULL | NULL | | 2 | Khilan | 1560 | 2009-11-20 00:00:00 | | 3 | kaushik | 3000 | 2009-10-08 00:00:00 | | 3 | kaushik | 1500 | 2009-10-08 00:00:00 | | 4 | Chaitali | 2060 | 2008-05-20 00:00:00 | | 5 | Hardik | NULL | NULL | | 6 | Komal | NULL | NULL | | 7 | Muffy | NULL | NULL | | 3 | kaushik | 3000 | 2009-10-08 00:00:00 | | 3 | kaushik | 1500 | 2009-10-08 00:00:00 | | 2 | Khilan | 1560 | 2009-11-20 00:00:00 | | 4 | Chaitali | 2060 | 2008-05-20 00:00:00 | +------+----------+--------+---------------------+
还有另外两个子句(即运算符),类似于UNION子句.
SQL INTERSECT Clause : 这用于组合两个SELECT语句,但仅返回第一个SELECT语句中与第二个SELECT语句中的行相同的行.
SQL EXCEPT子句 : 它组合了两个SELECT语句,并返回第一个SELECT语句中未被第二个SELECT语句返回的行.