经常遇到很多按照很多查询条件进行查询的情况，比如京东根据不同的条件筛选商品。其中经常出现很多条件不取值的情况，在后台应该如何完成最终的SQL语句呢？
如果采用JDBC进行处理，需要根据条件是否取值进行SQL语句的拼接，一般情况下是使用StringBuilder类及其append方法实现，还是有些繁琐的。如果你有使用 JDBC 或其它类似框架的经验，你就能体会到根据不同条件拼接 SQL语句的痛苦。例如拼接时要确保不能忘记添加必要的空格，还要注意去掉列表最后一个列名的逗号。利用动态 SQL 这一特性可以彻底摆脱这种痛苦。
MyBatis在简化操作方法提出了动态SQL功能，将使用Java代码拼接SQL语句，改变为在XML映射文件中截止标签拼接SQL语句。相比而言，大大减少了代码量，更灵活、高度可配置、利于后期维护。
MyBatis中动态SQL是编写在mapper.xml中的，其语法和JSTL类似，但是却是基于强大的OGNL表达式实现的。
MyBatis也可以在注解中配置SQL，但是由于注解功能受限，尤其是对于复杂的SQL语句，可读性很差，所以较少使用。
1.if标签
接口
public interface EmpMapper2 {
List<Emp> findByCondition(Emp emp);
}
SQL语句
<?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.msb.mapper.EmpMapper2">
<!--List<Emp> findByCondition(Emp emp);-->
<select id="findByCondition" resultType="emp">
select * from emp where 1=1
<if test="empno != null">
and empno =#{empno}
</if>
<if test="ename != null and ename != ''">
and ename like concat('%',#{ename},'%')
</if>
<if test="job != null and job != ''">
and job =#{job}
</if>
<if test="mgr != null">
and mgr =#{mgr}
</if>
<if test="hiredate != null">
and hiredate =#{hiredate}
</if>
<if test="sal != null">
and sal =#{sal}
</if>
<if test="comm != null">
and comm =#{comm}
</if>
<if test="deptno != null">
and deptno =#{deptno}
</if>
</select>
</mapper>
测试代码
public static void main(String[] args) {
SqlSession sqlSession = MyBatisUtil.getSqlSession(false);
EmpMapper2 mapper = sqlSession.getMapper(EmpMapper2.class);
Emp condition =new Emp();
/* condition.setDeptno(20);*/
/* condition.setSal(3000.0);*/
/*condition.setHiredate(new java.sql.Date(81,1,22));*/
condition.setComm(0.0);
condition.setDeptno(20);
List<Emp> emps = mapper.findEmpByCondition(condition);
for (Emp e:emps
) {
System.out.println(e);
}
}
2.where标签
用于处理where关键字和and
<select id="findEmpByCondition" resultType="emp">
select * from emp
<where>
<if test="empno != null">
and empno= #{empno}
</if>
<if test="ename != null and ename != ''">
and ename= #{ename}
</if>
<if test="job != null and job != ''">
and job= #{job}
</if>
<if test="mgr != null ">
and mgr= #{mgr}
</if>
<if test="hiredate != null ">
and hiredate= #{hiredate}
</if>
<if test="sal != null">
and sal= #{sal}
</if>
<if test="comm != null ">
and comm =#{comm}
</if>
<if test="deptno != null ">
and deptno= #{deptno}
</if>
</where>
</select>
Choose标签Choose标签CCCCCCCCCC
3.Choose标签
前面的when条件成立 后面的 when就不再判断了，和where标签的区别
<select id="findEmpByCondition2" resultType="emp">
select * from emp
<where>
<choose>
<when test="empno != null">
and empno= #{empno}
</when>
<when test="ename != null and ename != ''">
and ename= #{ename}
</when>
<when test="job != null and job != ''">
and job= #{job}
</when>
<when test="mgr != null ">
and mgr= #{mgr}
</when>
<when test="hiredate != null ">
and hiredate= #{hiredate}
</when>
<when test="sal != null">
and sal= #{sal}
</when>
<when test="comm != null ">
and comm =#{comm}
</when>
<when test="deptno != null ">
and deptno= #{deptno}
</when>
</choose>
</where>
</select>
4.Set标签
接口
int updateEmpByCondtion(Emp emp);
SQL语句
<!--int updateEmpByCondtion(Emp emp);-->
<update id="updateEmpByCondtion" >
update emp
<set>
<if test="ename != null and ename != '' ">
, ename =#{ename}
</if>
<if test="job != null and ename != '' ">
, job =#{job}
</if>
<if test="mgr != null ">
, mgr =#{mgr}
</if>
<if test="hiredate != null ">
, hiredate =#{hiredate}
</if>
<if test="sal != null ">
, sal =#{sal}
</if>
<if test="comm != null ">
, comm =#{comm}
</if>
<if test="deptno != null ">
, deptno =#{deptno}
</if>
</set>
where empno =#{empno}
</update>
5.Trim标签
Trim 标签处理 set
接口
int updateEmpByCondtion(Emp emp);
SQL语句
<update id="updateEmpByCondition2" >
update emp
<!--prefix 要增加什么前缀
prefixOverrides 要去除什么前缀
suffix 要增加什么后缀
suffixOverrides 要去除什么后缀
set 是trim的一种特殊情况
-->
<trim prefix="set" suffixOverrides="," >
<if test="ename != null and ename != ''">
ename= #{ename},
</if>
<if test="job != null and job != ''">
job= #{job},
</if>
<if test="mgr != null ">
mgr= #{mgr},
</if>
<if test="hiredate != null ">
hiredate= #{hiredate},
</if>
<if test="sal != null">
sal= #{sal},
</if>
<if test="comm != null ">
comm =#{comm},
</if>
<if test="deptno != null ">
deptno= #{deptno},
</if>
</trim>
where empno = #{empno}
</update>
测试代码
Trim标签 处理where
<select id="findEmpByCondition" resultType="emp">
select * from emp
<trim prefix="where" prefixOverrides="and">
<if test="empno != null">
and empno= #{empno}
</if>
<if test="ename != null and ename != ''">
and ename= #{ename}
</if>
<if test="job != null and job != ''">
and job= #{job}
</if>
<if test="mgr != null ">
and mgr= #{mgr}
</if>
<if test="hiredate != null ">
and hiredate= #{hiredate}
</if>
<if test="sal != null">
and sal= #{sal}
</if>
<if test="comm != null ">
and comm =#{comm}
</if>
<if test="deptno != null ">
and deptno= #{deptno}
</if>
</trim>
</select>
6.Bind标签
一般用于处理模糊查询的模板
接口
SQL语句
7.Sql标签
<sql id="empColumn">empno,ename,job,mgr,hiredate,sal,comm,deptno</sql>
<sql id="baseSelect">select <include refid="empColumn"></include> from emp</sql>
<!--List<Emp> findByCondition(Emp emp);-->
<select id="findByCondition" resultType="emp">
<include refid="baseSelect"></include>
<trim prefix="where" prefixOverrides="and">
<if test="empno != null">
and empno =#{empno}
</if>
<if test="ename != null and ename != ''">
<bind name="likePattern" value="'%'+ename+'%'"/>
and ename like #{likePattern}
</if>
<if test="job != null and job != ''">
and job =#{job}
</if>
<if test="mgr != null">
and mgr =#{mgr}
</if>
<if test="hiredate != null">
and hiredate =#{hiredate}
</if>
<if test="sal != null">
and sal =#{sal}
</if>
<if test="comm != null">
and comm =#{comm}
</if>
<if test="deptno != null">
and deptno =#{deptno}
</if>
</trim>
</select>
8.Foreach标签
接口
SQL语句
<!--List<Emp> findByEmpnos1(int[] empnos);
collection="" 遍历的集合或者是数组
参数是数组,collection中名字指定为array
参数是List集合,collection中名字指定为list
separator="" 多个元素取出的时候 用什么文字分隔
open="" 以什么开头
close="" 以什么结尾
item="" 中间变量名
for(Person per:PersonList)
-->
<select id="findByEmpnos1" resultType="emp">
select * from emp where empno in
<foreach collection="array" separator="," open="(" close=")" item="deptno">
#{deptno}
</foreach>
</select>
<!-- List<Emp> findByEmpnos2(List<Integer> empnos);-->
<select id="findByEmpnos2" resultType="emp">
select * from emp where empno in
<foreach collection="list" separator="," open="(" close=")" item="deptno">
#{deptno}
</foreach>
</select>
