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。
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:
链接:https://www.learnfk.comhttps://www.learnfk.com/derby/apache-derby-where-clause.html
来源:LearnFk无涯教程网
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
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); //获取Statement对象 Statement stmt=conn.createStatement(); //创建表结构,插入一些测试数据 String query="CREATE TABLE Employees(" + "Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, " + "Name VARCHAR(255), Salary INT NOT NULL, " + "Location VARCHAR(255), " + "PRIMARY KEY (Id))"; String query="INSERT INTO Employees(" + "Name, Salary, Location) VALUES " + "('Amit', 30000, 'Hyderabad'), " + "('Kalyan', 40000, 'Vishakhapatnam'), " + "('Renuka', 50000, 'Delhi'), " + "('Learnfk', 15000, 'Mumbai'), " + "('Trupthi', 45000, 'Kochin'), " + "('Suchatra', 33000, 'Pune'), " + "('Rahul', 39000, 'Lucknow'), " + "('Trupti', 45000, 'Kochin')"; //执行SQL语句 String query="SELECT * FROM Employees WHERE Salary>35000"; ResultSet rs=stmt.executeQuery(query); while(rs.next()) { System.out.println("Id: "+rs.getString("Id")); System.out.println("Name: "+rs.getString("Name")); System.out.println("Salary: "+rs.getString("Salary")); System.out.println("Location: "+rs.getString("Location")); System.out.println(" "); } } }
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
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