Mybatis学习三（动态sql语句）

动态sql语句主要为以下语句

1、动态SQL:if 语句
2、动态SQL:if+where 语句
3、动态SQL:if+set 语句
4、动态SQL:choose(when,otherwise) 语句
5、动态SQL:trim 语句
6、动态SQL: SQL 片段
7、动态SQL: foreach 语句

之前的几篇学习博客我用mybatis对一张表进行的CRUD操作，但是 SQL 语句都比较简单，如果有比较复杂的 SQL 语句，经常需要拼接，而拼接 SQL稍微不注意就容易出错此时我们可以用mybatis 动态SQL，通过 if, choose, when, otherwise, trim, where, set, foreach等标签来解决这个问题，这几个标签可组合成非常灵活的SQL语句，用起来也是很舒服

  1 <?xml version="1.0" encoding="UTF-8" ?>
  2 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  3 "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
  4 <!-- namespace:表示名称空间。现在的目的是区分id的. -->
  5 <mapper namespace="com.zhiyou100.zhl.dao.UsersDao">
  6     <sql id="detail">
  7         id,name,age,sex,day
  8     </sql>
  9     
 10     <select id="selByWhere" resultType="com.zhiyou100.zhl.bean.Users">
 11         select * from users
 12         <where>
 13             <if test="name!=null">
 14                 and name=#{name}
 15             </if>
 16             <if test="sex!=null and sex!=''">
 17                 and sex=#{sex}
 18             </if>
 19         </where>
 20     </select>
 21     
 22     <update id="updateWhere">
 23         update users
 24         <set>
 25             <if test="name!=null">
 26                 name=#{name},
 27             </if>
 28             <if test="sex!=null">
 29                 sex=#{sex},
 30             </if>
 31             <if test="age>0">
 32                 age=#{age},
 33             </if>
 34             <if test="day!=null">
 35                 day=#{day}
 36             </if>
 37         </set>
 38         where id=#{id}
 39     </update>
 40     
 41     <select id="selByWhere2" resultType="com.zhiyou100.zhl.bean.Users">
 42         select
 43         <include refid="detail"/>
 44         from users
 45         <where>
 46             <choose>
 47                 <when test="name!=null and name!=''">
 48                     name like concat('%',#{name},'%')
 49                 </when>
 50                 <when test="sex!=null and sex!=''">
 51                     sex=#{sex}
 52                 </when>
 53                 <otherwise>
 54                     age>=#{age}
 55                 </otherwise>
 56             </choose>
 57         </where>
 58     </select>
 59     
 60     <select id="selByWhere3" resultType="com.zhiyou100.zhl.bean.Users">
 61         select
 62         <include refid="detail"/>
 63         from users
 64         <trim prefix="where" prefixOverrides="and | or">
 65             <if test="name!=null and name!=''">
 66                 and name=#{name}
 67             </if>
 68             <if test="sex!=null and sex!=''">
 69                 and sex=#{sex}
 70             </if>
 71             <if test="age>0">
 72                 and age=#{age}
 73             </if>
 74             <if test="day!=null and day!=''">
 75                 and day=#{day}
 76             </if>
 77         </trim>
 78     </select>
 79     
 80     <update id="updateWhere2">
 81         update users
 82         <trim prefix="set" suffixOverrides=",">
 83             <if test="name!=null">
 84                 name=#{name},
 85             </if>
 86             <if test="sex!=null">
 87                 sex=#{sex},
 88             </if>
 89             <if test="age>0">
 90                 age=#{age},
 91             </if>
 92             <if test="day!=null">
 93                 day=#{day}
 94             </if>
 95         </trim>
 96         where id=#{id}
 97     </update>
 98     
 99     <delete id="deleteByIds">
100         delete from users
101         <where>
102             <foreach collection="ids" open="id in(" close=")" separator="," item="id">
103                 #{id}
104             </foreach>
105         </where>
106     </delete>
107     
108     
109     <delete id="deleteByIds2">
110         delete from users where id in
111         <foreach collection="ids" open="(" close=")" separator="," item="id">
112             #{id}
113         </foreach>
114     </delete>
115     
116     
117 </mapper>