9、Mybatis之动态SQL

9.1、环境搭建

9.1.1、创建新module

创建名为mybatis_dynamicSQL的新module，过程参考5.1节

9.1.2、创建Emp实体类

package org.rain.mybatis.pojo;

/**
 * @author liaojy
 * @date 2023/6/20 - 0:09
 */
public class Emp {
    private Integer empId;
    private String empName;
    private Integer age;
    private String gender;

    public Emp() {
    }

    public Emp(Integer empId, String empName, Integer age, String gender) {
        this.empId = empId;
        this.empName = empName;
        this.age = age;
        this.gender = gender;
    }

    public Integer getEmpId() {
        return empId;
    }

    public void setEmpId(Integer empId) {
        this.empId = empId;
    }

    public String getEmpName() {
        return empName;
    }

    public void setEmpName(String empName) {
        this.empName = empName;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    @Override
    public String toString() {
        return "Emp{" +
                "empId=" + empId +
                ", empName='" + empName + '\'' +
                ", age=" + age +
                ", gender='" + gender + '\'' +
                '}';
    }
}

9.1.3、创建Mapper接口和映射文件

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<?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="org.rain.mybatis.mapper.DynamicSQLMapper">

</mapper>

9.1.4、创建测试类

9.2、if标签

9.2.1、接口方法

List<Emp> getEmpsByCondition(Emp emp);

9.2.2、映射文件

    <!--List<Emp> getEmpsByCondition(Emp emp);-->
    <select id="getEmpsByCondition" resultType="Emp">
        select * from t_emp where
        <!--
            if标签：通过test属性中的表达式判断标签中的内容是否会拼接到sql中
        -->
        <if test="empName != '' and empName != null">
            emp_name = #{empName}
        </if>
        <if test="age != '' and age != null">
            and age = #{age}
        </if>
        <if test="gender != '' and gender != null">
            and gender = #{gender}
        </if>
    </select>

9.2.3、测试方法

模拟符合所有if标签的test属性中的表达式判断为true的情况

    @Test
    public void testCetEmpsByCondition(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper dynamicSQLMapper = sqlSession.getMapper(DynamicSQLMapper.class);
        Emp emp = new Emp();
        emp.setEmpName("张三");
        emp.setAge(20);
        emp.setGender("男");
        List<Emp> emps = dynamicSQLMapper.getEmpsByCondition(emp);
        System.out.println(emps);
        sqlSession.close();
    }

9.2.4、执行效果

所有if标签的内容都已顺利拼接到sql中

9.3、where标签

9.3.1、if标签缺陷

注意：当部分或全部if标签的test属性中的表达式判断为false时，sql可能会因为where条件的拼接报错

9.3.2、缺陷修正

9.3.2.1、方式一：增加恒成立条件

<!--List<Emp> getEmpsByCondition(Emp emp);-->
    <select id="getEmpsByCondition" resultType="Emp">
        select * from t_emp where 1=1
        <if test="empName != '' and empName != null">
            and emp_name = #{empName}
        </if>
        <if test="age != '' and age != null">
            and age = #{age}
        </if>
        <if test="gender != '' and gender != null">
            and gender = #{gender}
        </if>
    </select>

9.3.2.1、方式二：使用where标签

    <!--List<Emp> getEmpsByCondition(Emp emp);-->
    <select id="getEmpsByCondition" resultType="Emp">
        select * from t_emp
        <!--
            where标签的效果：
            1、若标签中有条件成立，则自动生成where关键字，且能自动删除（前方）多余的and关键字
            2、若标签中没有任何条件成立，则没有任何功能
        -->
        <where>
            <if test="empName != '' and empName != null">
                and emp_name = #{empName}
            </if>
            <if test="age != '' and age != null">
                and age = #{age}
            </if>
            <if test="gender != '' and gender != null">
                and gender = #{gender}
            </if>
        </where>
    </select>

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9.4、trim标签

    <!--List<Emp> getEmpsByCondition(Emp emp);-->
    <select id="getEmpsByCondition" resultType="Emp">
        select * from t_emp
        <!--
            trim标签：用于添加或去掉标签中的内容
                prefix属性：在标签中内容的前面添加指定的内容
                suffix属性：在标签中内容的后面添加指定的内容
                prefixOverrides属性：在标签中内容的前面去掉指定的内容
                suffixOverrides属性：在标签中内容的后面去掉指定的内容
        -->
        <trim prefix="where" suffixOverrides="and">
            <if test="empName != '' and empName != null">
                emp_name = #{empName} and
            </if>
            <if test="age != '' and age != null">
                age = #{age} and
            </if>
            <if test="gender != '' and gender != null">
                gender = #{gender} and
            </if>
        </trim>
    </select>

9.5、choose标签组

    <!--List<Emp> getEmpsByCondition(Emp emp);-->
    <select id="getEmpsByCondition" resultType="Emp">
        select * from t_emp
        <where>
            <!--
                choose、when和otherwise标签组合（少用）：
                when（相当于if...else）至少有一个，otherwise（相当于else）至多有一个
            -->
            <choose>
                <when test="empName != '' and empName != null">
                    emp_name = #{empName}
                </when>
                <when test="age != '' and age != null">
                    age = #{age}
                </when>
                <when test="gender != '' and gender != null">
                    gender = #{gender}
                </when>
            </choose>
        </where>
    </select>

9.6、foreach标签

9.6.1、批量添加

9.6.1.1、接口方法

void insertMoreEmp(@Param("emps") List<Emp> emps);

9.6.1.2、映射文件

    <!--void insertMoreEmp(@Param("emps") List<Emp> emps);-->
    <!--
        foreach标签：
            collection属性：设置要循环的数组或集合
            item属性：设置表示数组或集合中迭代元素的变量名
            separator属性：设置每次foreach循环之间的分隔符
    -->
    <insert id="insertMoreEmp">
        insert into t_emp values
        <foreach collection="emps" item="emp" separator=",">
            (null ,#{emp.empName},#{emp.age},#{emp.gender},null)
        </foreach>
    </insert>

9.6.1.3、测试方法

    @Test
    public void testInsertMoreEmp(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper dynamicSQLMapper = sqlSession.getMapper(DynamicSQLMapper.class);
        Emp emp1 = new Emp(null,"小明1",20,"男");
        Emp emp2 = new Emp(null,"小明2",20,"男");
        Emp emp3 = new Emp(null,"小明3",20,"男");
        List<Emp> emps = Arrays.asList(emp1, emp2, emp3);
        dynamicSQLMapper.insertMoreEmp(emps);
        sqlSession.close();
    }

9.6.1.4、执行效果

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9.6.2、批量删除

9.6.2.1、接口方法

void deleteMoreEmp(@Param("empIds") Integer[] empIds);

9.6.2.2、映射文件

    <!--void deleteMoreEmp(@Param("empIds") Integer[] empIds);-->
    <!--
        foreach标签：
            collection属性：设置要循环的数组或集合
            item属性：设置表示数组或集合中迭代元素的变量名
            separator属性：设置每次foreach循环之间的分隔符
            open属性：设置foreach循环体的前缀
            close属性：设置foreach循环体的后缀
    -->
    <delete id="deleteMoreEmp">
        delete from t_emp where emp_id in
        <foreach collection="empIds" item="empId" separator="," open="(" close=")">
            #{empId}
        </foreach>
    </delete>

9.6.2.3、测试方法

    @Test
    public void testDeleteMoreEmp(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper dynamicSQLMapper = sqlSession.getMapper(DynamicSQLMapper.class);
        Integer[] empIds = {6, 7};
        dynamicSQLMapper.deleteMoreEmp(empIds);
        sqlSession.close();
    }

9.6.2.4、执行效果

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9.6.2.5、映射文件（变体）

    <delete id="deleteMoreEmp">
        delete from t_emp where
        <foreach collection="empIds" item="empId" separator="or">
            emp_id = #{empId}
        </foreach>
    </delete>

9.7、sql标签

    <!--
        sql标签：设置一个sql片段
    -->
    <sql id="empColumns">
        emp_id,emp_name,age,gender
    </sql>

    <!--List<Emp> getEmpsByCondition(Emp emp);-->
    <!--
        include标签：
            refid属性：通过sql标签的id引用相关sql片段
    -->
    <select id="getEmpsByCondition" resultType="Emp">
        select <include refid="empColumns"></include> from t_emp
    </select>