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Sql学习第一天——SQLUNION和UNIONALL操作符认识 - 站长搜索
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Sql学习第一天——SQLUNION和UNIONALL操作符认识

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2022-09-23 17:06:38 转载来源: 网络整理/侵权必删

关于SQLUNION操作符UNION操作符用于合并两个或多个SELECT语句的结果集。注意:1.UNION内部的SELECT语句必须拥有相同数量的列

关于SQL UNION 操作
UNION 操作符用于合并两个或多个 SELECT 语句的结果集。
注意
1.UNION 内部的 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时,每条 SELECT 语句中的列的顺序必须相同。
2.UNION 结果集中的列名总是等于 UNION 中第一个 SELECT 语句中的列名。
SQL UNION 语法(结果集没有重复值):
复制代码 代码如下:

SELECT s FROM table1
UNION
SELECT s FROM table2

SQL UNION ALL 语法(结果集有重复的值):
复制代码 代码如下:

SELECT s FROM table1
UNION ALL
SELECT s FROM table2

例子表:
 表一(Employees_China):
    E_ID E_Name
    01    Zhang, Hua
    02    Wang, Wei
    03    Carter, Thomas
    04    Yang, Ming
 表二(Employees_USA):
    E_ID   E_Name
    01    Adams, John
    02    Bush, George
    03    Carter, Thomas
    04    Gates, Bill
实例:
关于UNION: 
复制代码 代码如下:
 
SELECT E_Name FROM Employees_China
  UNION
  SELECT E_Name FROM Employees_USA

结果如下(去除了重复数据):
E_Name
Zhang, Hua
Wang, Wei
Carter, Thomas
Yang, Ming
Adams, John
Bush, George
Gates, Bill

关于UNION ALL:
复制代码 代码如下:

  SELECT E_Name FROM Employees_China   
  UNION ALL

  SELECT E_Name FROM Employees_USA

 结果如下(还保留了重复的数据):
E_Name
Zhang, Hua
Wang, Wei
Carter, Thomas
Yang, Ming
Adams, John
Bush, George
Carter, Thomas
Gates, Bill

标签: Sql 学习 第一天 SQLUNION UNIONALL 操作 认识


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