目录

<sql:param>

《sql:param》标记用作《sql:query》标记的嵌套操作, 《sql:update》标记用于为值占位符提供值。 如果提供了null值,则占位符的值设置为SQL NULL

属性 (Attribute)

《sql:param》标记具有以下属性 -

属性 描述 需要 默认
Value 要设置的参数的值 NoBody

例子 (Example)

要从基本概念开始,让我们在TEST数据库中创建一个Employees表,并在该表中创建几条记录,如下所示 -

Step 1

打开Command Prompt并切换到安装目录,如下所示 -

C:\>
C:\>cd Program Files\MySQL\bin
C:\Program Files\MySQL\bin>

Step 2

登录数据库如下 -

C:\Program Files\MySQL\bin>mysql -u root -p
Enter password: ********
mysql>

Step 3

TEST数据库中创建Employee表,如下所示 -

mysql> use TEST;
mysql> create table Employees
   (
      id int not null,
      age int not null,
      first varchar (255),
      last varchar (255)
   );
Query OK, 0 rows affected (0.08 sec)
mysql>

创建数据记录 (Create Data Records)

我们现在将在Employee表中创建一些记录,如下所示 -

mysql> INSERT INTO Employees VALUES (100, 18, 'Zara', 'Ali');
Query OK, 1 row affected (0.05 sec)
mysql> INSERT INTO Employees VALUES (101, 25, 'Mahnaz', 'Fatma');
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO Employees VALUES (102, 30, 'Zaid', 'Khan');
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO Employees VALUES (103, 28, 'Sumit', 'Mittal');
Query OK, 1 row affected (0.00 sec)
mysql>

现在让我们编写一个JSP,它将利用《sql:update》标签来执行SQL DELETE语句,从表中删除id = 103一条记录,如下所示 -

<%@ page import = "java.io.*,java.util.*,java.sql.*"%>
<%@ page import = "javax.servlet.http.*,javax.servlet.*" %>
<%@ taglib uri = "http://java.sun.com/jsp/jstl/core" prefix = "c"%>
<%@ taglib uri = "http://java.sun.com/jsp/jstl/sql" prefix = "sql"%>
<html>
   <head>
      <title>JSTL sql:param Tag</title>
   </head>
   <body>
      <sql:setDataSource var = "snapshot" driver = "com.mysql.jdbc.Driver"
         url = "jdbc:mysql://localhost/TEST"
         user = "root"  password = "pass123"/>
      <c:set var = "empId" value = "103"/>
      <sql:update dataSource = "${snapshot}" var = "count">
         DELETE FROM Employees WHERE Id = ?
         <sql:param value = "${empId}" />
      </sql:update>
      <sql:query dataSource = "${snapshot}" var = "result">
         SELECT * from Employees;
      </sql:query>
      <table border = "1" width = "100%">
         <tr>
            <th>Emp ID</th>
            <th>First Name</th>
            <th>Last Name</th>
            <th>Age</th>
         </tr>
         <c:forEach var = "row" items = "${result.rows}">
            <tr>
               <td> <c:out value = "${row.id}"/></td>
               <td> <c:out value = "${row.first}"/></td>
               <td> <c:out value = "${row.last}"/></td>
               <td> <c:out value = "${row.age}"/></td>
            </tr>
         </c:forEach>
      </table>
   </body>
</html>

访问上面的JSP,将显示以下结果 -

+-------------+----------------+-----------------+-----------------+
|    Emp ID   |    First Name  |     Last Name   |       Age       |
+-------------+----------------+-----------------+-----------------+
|     100     |    Zara        |     Ali         |       18        |
|     101     |    Mahnaz      |     Fatma       |       25        |
|     102     |    Zaid        |     Khan        |       30        |
+-------------+----------------+-----------------+-----------------+

您可以使用SQL UPDATESELECT语句尝试《sql:param》标记,就像我们在DELETE语句中使用它一样。

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