This article brings you relevant knowledge about mysql, which mainly introduces the relevant content about dynamic SQL splicing. In actual business development, our SQL statements are usually dynamically spliced. , such as the SQL statement of the conditional search function, etc. Let’s take a look at it together. I hope it will be helpful to everyone.
Recommended learning: mysql video tutorial
In the previous learning process, the SQL statements we used were very simple. In actual business development, our SQL statements are usually spliced dynamically, such as SQL statements for conditional search functions.
# 提供了一个功能：用户可以在页面上根据username、sex、address进行搜索 # 用户输入的搜索条件：可以是一个条件，也可能是两个、三个 # 只输入一个条件：姓名是"王" SELECT * FROM USER WHERE username LIKE '%王%' # 只输入一个条件：性别是“男” SELECT * FROM USER WHERE sex = '男' # 输入两个条件：姓名“王”，性别“男” SELECT * FROM USER WHERE username LIKE '%王%' AND sex = '男' # 输入三个条件：姓名“王”，性别“男”，地址“北京” SELECT * FROM USER WHERE username LIKE '%王%' AND sex = '男' AND address LIKE '%北京%';
In Mybatis, SQL statements are written in the XML file of the mapping configuration. Mybatis provides some XML tags to implement dynamic SQL splicing.
Commonly used tags are:
: used for judgment, equivalent to if judgment in Java
: Usually used in conjunction with if to replace where 1=1in (value1, value2, ...)<if test="判断条件，使用OGNL表达式进行判断"> SQL语句内容， 如果判断为true，这里的SQL语句就会进行拼接</if>
package com.demo.dao;import com.demo.domain.User;import java.util.List;public interface UserDao {
/**
* 根据username和sex搜索用户
* @param user 封装了搜索条件的User对象
* @return 搜索的结果
*/
List<User> search1(User user);}<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.itheima.dao.UserDao">
<!--
if标签：用于条件判断
语法：<if test="用OGNL表达式判断"> 如果判断为true，这里的内容会拼接上去 </if>
注意：标签里写OGNL表达式，不要再加#{}、${}
常用的OGNL表达式：
比较：>, <, >=, <=, ==, != 或者 gt, lt, gte, lte, eq, neq
逻辑：&&,||,! 或者 and, or, not
调用方法：username.length(), list.size()
-->
<select id="search1" resultType="User">
select * from user where 1=1 <if test="username != null and username.length()>0">
and username like "%"#{username}"%" </if>
<if test="sex != null and sex.length()>0">
and sex = #{sex} </if>
</select></mapper>package com.demo;import com.demo.dao.UserDao;import com.demo.domain.User;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.After;import org.junit.Before;import org.junit.Test;import java.io.IOException;import java.io.InputStream;import java.util.List;public class SqlTest {
private UserDao userDao;
private SqlSession session;
private InputStream is;
/**
* 要求：根据username和sex搜索用户
* 搜索条件放到user对象里
*/
@Test
public void testSearch(){
User user = new User();
// user.setUsername("王");
// user.setSex("男");
List<User> userList = userDao.search1(user);
userList.forEach(System.out::println);
}
@Before
public void init() throws IOException {
//1. 读取全局配置文件
is = Resources.getResourceAsStream("SqlMapConfig.xml");
//2. 得到一个SqlSession对象
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
session = factory.openSession();
userDao = session.getMapper(UserDao.class);
}
@After
public void destroy() throws IOException {
session.close();
is.close();
}}. If you do not write it, the SQL statement will have syntax errors. Mybatis provides a technology that replaces where 1=1: <where></where> tag.
tag insteadwhere 1=1
/** * 根据username和sex搜索用户 * @param user 封装了搜索条件的User对象 * @return 搜索的结果 */List<User> search1(User user);
<!--
where标签：让Mybatis帮我们生成一个where关键字
Mybatis会智能判断：
如果一个条件都没有，就不生成where关键字
如果有条件，会判断是否有多余的and关键字，把多余的and去掉
注意：建议把所有的where条件都放到where标签里边
--><select id="search1" resultType="User">
select * from user <where>
<if test="username != null and username.length()>0">
and username like "%"#{username}"%" </if>
<if test="sex != null and sex.length()>0">
and sex = #{sex} </if>
</where></select>
@Testpublic void testSearch(){
User user = new User();
// user.setUsername("王");
// user.setSex("男");
List<User> userList = userDao.search1(user);
userList.forEach(System.out::println);}select * from user where id = 1 or id = 2 or id = 3;select * from user where id in (1, 2, 3);
If we pass a collection of IDs, how do we traverse the collection and splice the SQL statement in the mapping file? This can be achieved using the
foreach tag. <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"><!--
foreach标签：
属性：
collection：被循环遍历的对象，使用OGNL表达式获取，注意不要加#{}
open：循环之前，拼接的SQL语句的开始部分
item：定义变量名，代表被循环遍历中每个元素，生成的变量名
separator：分隔符
close：循环之后，拼接SQL语句的结束部分
标签体：
使用#{OGNL}表达式，获取到被循环遍历对象中的每个元素
--><foreach collection="" open="id in(" item="id" separator="," close=")">
#{id}</foreach></pre><div class="contentsignin">Copy after login</div></div>
package com.itheima.domain;public class QueryVO {
private Integer[] ids;
public Integer[] getIds() {
return ids;
}
public void setIds(Integer[] ids) {
this.ids = ids;
}}/** * QueryVO里有一个Integer[] ids * 要求：根据ids查询对应的用户列表 */List<User> search2(QueryVO vo);
<!--
foreach标签：用于循环遍历
collection：被循环的集合/数组
item：定义一个变量
separator：定义拼接时的分隔符
open：拼接字符串时的开始部分
close：拼接字符串时的结束部分
相当于 for(Integer id: ids){}
select * from user where id in(41, 42, 45)
-->
<select id="search2" resultType="User">
<!--select * from user where id in(41, 42, 45)-->
select * from user where <foreach collection="ids" open="id in(" item="id" separator="," close=")">
#{id} </foreach>
</select> @Test
public void testSearch2(){
QueryVO vo = new QueryVO();
vo.setIds(new Integer[]{41,42,43,44,45});
List<User> userList = userDao.search2(vo);
userList.forEach(System.out::println);
}. Mybatis provides a <sql> tag to extract repeated SQL fragments and can be reused. <h5>
<a id="_326"></a>语法介绍</h5>
<p>在映射文件中定义SQL片段：</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"><sql id="唯一标识">sql语句片段</sql></pre><div class="contentsignin">Copy after login</div></div>
<p>在映射文件中引用SQL片段：</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"><include refid="sql片段的id"></include></pre><div class="contentsignin">Copy after login</div></div>
<h5>
<a id="_340"></a>使用示例</h5>
<p>在查询用户的SQL中，需要重复编写：<code>select * from user。把这部分SQL提取成SQL片段以重复使用
修改QueryVO，增加成员变量user
package com.itheima.domain;/**
* @author liuyp
* @date 2021/09/07
*/public class QueryVO {
private Integer[] ids;
private User user;
//get/set方法……}在映射器UserDao里加方法
/** * 动态SQL拼接的综合应用：if、where、foreach * 要求：QueryVo里有ids、username、sex值，根据这些值进行搜索 */ List<User> search3(QueryVO vo);
在映射文件UserDao.xml里配置statement
<select id="search3" resultType="User">
<!--select * from user-->
<include refid="selUser"/>
<where>
<if test="ids != null and ids.length > 0">
<foreach collection="ids" open="and id in(" item="id" separator="," close=")">
#{id} </foreach>
</if>
<!--<if test="user != null">
<if test="user.username != null and user.username.length() > 0">
and username like "%"#{user.username}"%"
</if>
<if test="user.sex != null and user.sex.length() > 0">
and sex = #{user.sex}
</if>
</if>-->
<include refid="userCondition"/>
</where></select><!--
sql标签：用于定义一个sql片段
include标签：什么时候要引用某个SQL片段，就使用include标签
注意：引入SQL片段之后，最终的SQL语句必须要完全符合语法
--><sql id="selUser">select * from user</sql><sql id="userCondition">
<if test="user != null">
<if test="user.username != null and user.username.length() > 0">
and username like "%"#{user.username}"%" </if>
<if test="user.sex != null and user.sex.length() > 0">
and sex = #{user.sex} </if>
</if></sql>在测试类里加测试方法
@Test
public void testSearch3(){
QueryVO vo = new QueryVO();
vo.setIds(new Integer[]{41,42,43,44,45});
// User user = new User();
// user.setUsername("王");
// user.setSex("男");
// vo.setUser(user);
List<User> userList = userDao.search3(vo);
userList.forEach(System.out::println);
}推荐学习：mysql视频教程
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