无涯教程-Derby - Where语句 WHERE子句用于SELECT，DELETE或UPDATE语句中，以指定需要在

WHERE子句用于SELECT，DELETE或UPDATE语句中，以指定需要在其上执行操作的行，通常，此子句后面是返回布尔值的条件或表达式，仅对满足给定条件的行执行选择，删除或更新操作。

ij> SELECT * from table_name WHERE condition;
or,
ij> DELETE from table_name WHERE condition;
or,
ij> UPDATE table_name SET column_name=value WHERE condition;

WHERE子句可以使用比较运算符，如=，!= 、、 =以及BETWEEN和LIKE运算符。

Where - 命令行示例

让无涯教程假设无涯教程在数据库中有一个名为Employees的表，具有7条记录，如下所示-

ID |NAME |SALARY |LOCATION
-----------------------------------------------------------------------------
1 |Amit |30000 |Hyderabad
2 |Kalyan |40000 |Vishakhapatnam
3 |Renuka |50000 |Delhi
4 |Learnfk |15000 |Mumbai
5 |Trupthi |45000 |Kochin
6 |Suchatra |33000 |Pune
7 |Rahul |39000 |Lucknow

以下SQL DELETE语句获取薪水超过35000的员工的记录-

ij> SELECT * FROM Employees WHERE Salary>35000;

这将产生以下输出-

ID |NAME |SALARY |LOCATION 
---------------------------------------------------
2 |Kalyan |40000 |Vishakhapatnam
3 |Renuka |50000 |Delhi
5 |Trupthi |45000 |Kochin
7 |Rahul |39000 |Lucknow
4 rows selected

同样，您也可以使用此子句删除和更新记录。

以下示例更新了薪水少于30000的人员的位置。

ij> UPDATE Employees SET Location=Vijayawada WHERE Salary<35000;
3 rows inserted/updated/deleted

如果您验证表的内容，则可以看到更新后的表，如下所示:

ij> SELECT * FROM Employees;
ID |NAME |SALARY |LOCATION
------------------------------------------------------------------------------
1 |Amit |30000 |Vijayawada
2 |Kalyan |40000 |Vishakhapatnam
3 |Renuka |50000 |Delhi
4 |Learnfk |15000 |Vijayawada
5 |Trupthi |45000 |Kochin
6 |Suchatra |33000 |Vijayawada
7 |Rahul |39000 |Lucknow
7 rows selected

Where - JDBC示例

以下JDBC示例演示了如何使用WHERE子句并使用JDBC程序在Apache Derby中的表上执行CURD操作，在这里，无涯教程使用嵌入式驱动程序连接到名为sampleDB的数据库(如果不存在则创建)。

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
public class WhereClauseExample {
   public static void main(String args[]) throws Exception {
      //注册驱动
      Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
      //创建连接
      String URL="jdbc:derby:sampleDB;create=true";
      Connection conn=DriverManager.getConnection(URL);
  </span><span class="com">//获取Statement对象</span><span class="pln">
  </span><span class="typ">Statement</span><span class="pln"> stmt</span><span class="pun">=</span><span class="pln">conn</span><span class="pun">.</span><span class="pln">createStatement</span><span class="pun">();</span><span class="pln">

  </span><span class="com">//创建表结构,插入一些测试数据</span><span class="pln">
  </span><span class="typ">String</span><span class="pln"> query</span><span class="pun">=</span><span class="str">"CREATE TABLE Employees("</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"Name VARCHAR(255), Salary INT NOT NULL, "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"Location VARCHAR(255), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"PRIMARY KEY (Id))"</span><span class="pun">;</span><span class="pln">
  </span><span class="typ">String</span><span class="pln"> query</span><span class="pun">=</span><span class="str">"INSERT INTO Employees("</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"Name, Salary, Location) VALUES "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Amit, 30000, Hyderabad), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Kalyan, 40000, Vishakhapatnam), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Renuka, 50000, Delhi), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Learnfk, 15000, Mumbai), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Trupthi, 45000, Kochin), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Suchatra, 33000, Pune), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Rahul, 39000, Lucknow), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Trupti, 45000, Kochin)"</span><span class="pun">;</span><span class="pln">
  </span><span class="com">//执行SQL语句</span><span class="pln">
  </span><span class="typ">String</span><span class="pln"> query</span><span class="pun">=</span><span class="str">"SELECT * FROM Employees WHERE Salary&gt;35000"</span><span class="pun">;</span><span class="pln">
  </span><span class="typ">ResultSet</span><span class="pln"> rs</span><span class="pun">=</span><span class="pln">stmt</span><span class="pun">.</span><span class="pln">executeQuery</span><span class="pun">(</span><span class="pln">query</span><span class="pun">);</span><span class="pln">
  </span><span class="kwd">while</span><span class="pun">(</span><span class="pln">rs</span><span class="pun">.</span><span class="kwd">next</span><span class="pun">())</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
     </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Id: "</span><span class="pun">+</span><span class="pln">rs</span><span class="pun">.</span><span class="pln">getString</span><span class="pun">(</span><span class="str">"Id"</span><span class="pun">));</span><span class="pln">
     </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Name: "</span><span class="pun">+</span><span class="pln">rs</span><span class="pun">.</span><span class="pln">getString</span><span class="pun">(</span><span class="str">"Name"</span><span class="pun">));</span><span class="pln">
     </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Salary: "</span><span class="pun">+</span><span class="pln">rs</span><span class="pun">.</span><span class="pln">getString</span><span class="pun">(</span><span class="str">"Salary"</span><span class="pun">));</span><span class="pln">
     </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Location: "</span><span class="pun">+</span><span class="pln">rs</span><span class="pun">.</span><span class="pln">getString</span><span class="pun">(</span><span class="str">"Location"</span><span class="pun">));</span><span class="pln">
     </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">" "</span><span class="pun">);</span><span class="pln">
  </span><span class="pun">}</span><span class="pln">

}
}

在执行上述程序时，您将获得以下输出-

Id: 2
Name: Kalyan
Salary: 43000
Location: Chennai
Id: 3
Name: Renuka
Salary: 50000
Location: Delhi
Id: 5
Name: Trupthi
Salary: 45000
Location: Kochin
Id: 7
Name: Rahul
Salary: 39000
Location: Lucknow

无涯教程-Derby - Where语句

Where - 命令行示例

Where - JDBC示例

参考链接