MyBatis的使用八(动态SQL)

本主要讲述mybatis处理动态sql语句

一. 问题引入

　　前端展示的数据表格中，查询条件可能不止一个，如何将用户输入的多个查询条件，拼接到sql语句中呢？

　　DynamicMapper接口声明如下

public interface DynamicMapper {

    // 动态查询员工信息
    List<Employee> selectEmpDynamic(@Param("emp") Employee emp);
}

　　如何在DynamicMapper.xml文件中，编写sql语句，使其动态查询呢？

二. 动态SQL查询

　　1. <if>标签

　　DynamicMapper.xml文件声明如下

    <!--// 动态查询员工信息
    List<Employee> selectEmpDynamic(Employee emp);-->
    <select id="selectEmpDynamic" resultType="Employee">
        select * from t_emp where 1=1
        <if test="emp.empName != null and emp.empName != '' ">
            and emp_name = #{emp.empName}
        </if>
        <if test="emp.age != null and emp.age != '' ">
            and age = #{emp.age}
        </if>
        <if test="emp.gender != null and emp.gender != '' ">
            and gender = #{emp.gender}
        </if>
    </select>

　　测试test

    @Test
    // 测试动态查询
    public void test01(){
        SqlSession sqlSession = SQLSessionUtils.getSqlSession();
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        Employee emp = new Employee(null, "", 23, "");
        List<Employee> list = mapper.selectEmpDynamic(emp);
        System.out.println(list);
    }

　　运行结果如下

DEBUG 02-05 13:49:28,390 ==>  Preparing: select * from t_emp where 1=1 and age = ? (BaseJdbcLogger.java:137)
DEBUG 02-05 13:49:28,432 ==> Parameters: 23(Integer) (BaseJdbcLogger.java:137)
DEBUG 02-05 13:49:28,461 <==      Total: 1 (BaseJdbcLogger.java:137)
[Employee{empId=1, empName='张三', age=23, gender='男', dept=null}]

　　注意：单独使用 < if > 标签时，需要在where后面添加恒成立的条件，例如 1=1，之后在< if >标签中的test写判断条件

　　2. <where> + <if>标签

　　DynamicMapper.xml文件声明如下

    <select id="selectEmpDynamicTwo" resultType="Employee">
        select * from t_emp
        <where>
            <if test="emp.empName != null and emp.empName != '' ">
                and emp_name = #{emp.empName}
            </if>
            <if test="emp.age != null and emp.age != '' ">
                and age = #{emp.age} and
            </if>
            <if test="emp.gender != null and emp.gender != '' ">
                and gender = #{emp.gender}
            </if>
        </where>
    </select>

　　测试test1

    @Test
    // 测试动态查询
    public void test01(){
        SqlSession sqlSession = SQLSessionUtils.getSqlSession();
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        Employee emp = new Employee(null, "张三", 23, "");
        List<Employee> list = mapper.selectEmpDynamic(emp);
        System.out.println(list);
    }

　　test1 运行结果如下

DEBUG 02-05 14:00:47,159 ==>  Preparing: select * from t_emp WHERE emp_name = ? and age = ? (BaseJdbcLogger.java:137)
DEBUG 02-05 14:00:47,190 ==> Parameters: 张三(String), 23(Integer) (BaseJdbcLogger.java:137)
DEBUG 02-05 14:00:47,211 <==      Total: 1 (BaseJdbcLogger.java:137)
[Employee{empId=1, empName='张三', age=23, gender='男', dept=null}]

　　注意：<where>标签只会在至少有一个子元素的条件返回 SQL 子句的情况下才去插入“WHERE”子句。而且，若语句的开头为“AND”或“OR”，<where>标签也会将它们去除。

　　但是当DynamicMapper.xml声明如下

    <select id="selectEmpDynamic" resultType="Employee">
        select * from t_emp
        <where>
            <if test="emp.empName != null and emp.empName != '' ">
                emp_name = #{emp.empName} and
            </if>
            <if test="emp.age != null and emp.age != '' ">
                age = #{emp.age} and
            </if>
            <if test="emp.gender != null and emp.gender != '' ">
                gender = #{emp.gender}
            </if>
        </where>
    </select>

　　测试test2

    @Test
    // 测试动态查询
    public void test01(){
        SqlSession sqlSession = SQLSessionUtils.getSqlSession();
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        Employee emp = new Employee(null, "张三", 23, "");
        List<Employee> list = mapper.selectEmpDynamic(emp);
        System.out.println(list);
    }

　　test2 运行结果如下

DEBUG 02-05 14:08:29,175 ==>  Preparing: select * from t_emp WHERE emp_name = ? and age = ? and (BaseJdbcLogger.java:137)
DEBUG 02-05 14:08:29,206 ==> Parameters: 张三(String), 23(Integer) (BaseJdbcLogger.java:137) 

　　sql语句出现错误，即<where>标签无法去除SQL子句后面的and或者or。

　　3. <trim> + <if> 标签

　　DynamicMapper.xml文件声明如下

     <select id="selectEmpDynamic" resultType="Employee">
        select * from t_emp
        <!--prefix="where" 前缀添加where
        suffix="where" 后缀添加where
        prefixOverrides="and" 前缀删除and
        suffixOverrides="and" 后缀删除and-->
        <trim prefix="where" suffixOverrides="and">
            <if test="emp.empName != null and emp.empName != '' ">
                emp_name = #{emp.empName} and
            </if>
            <if test="emp.age != null and emp.age != '' ">
                age = #{emp.age} and
            </if>
            <if test="emp.gender != null and emp.gender != '' ">
                gender = #{emp.gender}
            </if>
        </trim>
    </select>

　　注意：<trim>标签的功能比<where>标签的功能更加丰富

　　test2运行结果如下

DEBUG 02-05 14:14:51,893 ==>  Preparing: select * from t_emp where emp_name = ? and age = ? (BaseJdbcLogger.java:137)
DEBUG 02-05 14:14:51,924 ==> Parameters: 张三(String), 23(Integer) (BaseJdbcLogger.java:137)
DEBUG 02-05 14:14:51,943 <==      Total: 1 (BaseJdbcLogger.java:137)
[Employee{empId=1, empName='张三', age=23, gender='男', dept=null}]

　　完美解决and在SQL子句后面的问题。

三. 批量增加【删除】

　　1. 批量增加

　　DynamicMapper接口声明如下

public interface DynamicMapper {

    // 批量添加emp对象
    int insertEmpList(@Param("emps") List<Employee> emps);
}

　　DynamicMapper.xml文件声明如下

    <!--// 批量添加emp对象
    int insertEmpList(@Param("emps") List<Employee> emps);-->
    <insert id="insertEmpList" >
        insert into t_emp values
        <foreach collection="emps" item="emp" separator=",">
            (null,#{emp.empName},#{emp.age},#{emp.gender},null)
        </foreach>
    </insert>

　　2. 批量删除

　　DynamicMapper接口声明如下

public interface DynamicMapper {

    // 批量删除emp对象
    int deleteEmpList(@Param("empIds") Integer[] empIds);
}

　　DynamicMapper.xml文件声明如下

    <!--// 批量删除emp对象-->
    <!--int deleteEmpList(@Param("empIds") Integer[] empIds);-->
    <!--方式1: -->
    <delete id="deleteEmpListOne" >
        delete from t_emp
        where emp_id in
        (
            <foreach collection="empIds" item="empId" separator=",">
                #{empId}
            </foreach>
            )
    </delete>

    <!--方式2: -->
    <delete id="deleteEmpListTwo" >
        delete from t_emp
        where emp_id in
        <foreach collection="empIds" item="empId" separator="," open="(" close=")">
            #{empId}
        </foreach>
    </delete>

    <!--方式3: -->
    <delete id="deleteEmpList" >
        delete from t_emp
        where
        <foreach collection="empIds" item="empId" separator="or">
            emp_id = #{empId}
        </foreach>
    </delete>

　　注意：< foreach >标签的使用，

　　　　collecion：传入的集合【数组】名；

　　　　item：集合【数组】中元素的类型；

　　　　separator：以指定字符串为分隔符；

　　　　open：以指定字符串开始，close：以指定字符串结尾。