MyBatis 动态 SQL的使用场景与实现方法

SELECT * FROM user WHERE username = 'Tom';

SELECT * FROM user;

SELECT * FROM user WHERE username = 'Tom';

String sql = "select * from user where 1=1";
if(username != null){
    sql += " and username = #{username}";
}
if(age != null){
    sql += " and age = #{age}";
}

CREATE TABLE user(
    id INT PRIMARY KEY AUTO_INCREMENT,
    username VARCHAR(50),
    age INT,
    gender VARCHAR(10)
);

public class User {
    private Integer id;
    private String username;
    private Integer age;
    private String gender;
    // getter/setter
}

public interface UserMapper {
    List<User> select(User user);
}

<select id="select" resultType="User">
    SELECT * FROM user
    WHERE 1=1
    <if test="username != null">
        AND username = #{username}
    </if>
    <if test="age != null">
        AND age = #{age}
    </if>
</select>

User user = new User();
user.setUsername("Tom");

SELECT * FROM user
WHERE 1=1
AND username = 'Tom'

User user = new User();
user.setAge(20);

SELECT * FROM user
WHERE 1=1
AND age = 20

WHERE 1=1

<select id="select" resultType="User">
    SELECT * FROM user
    <where>
        <if test="username != null">
            AND username = #{username}
        </if>
        <if test="age != null">
            AND age = #{age}
        </if>
    </where>
</select>

SELECT * FROM user

SELECT * FROM user
WHERE username = 'Tom'

<update id="updateUser">
    UPDATE user
    <set>
        <if test="username != null">
            username = #{username},
        </if>
        <if test="age != null">
            age = #{age},
        </if>
        <if test="gender != null">
            gender = #{gender},
        </if>
    </set>
    WHERE id = #{id}
</update>

UPDATE user
SET username = 'Tom',
    age = 20
WHERE id = 1

SELECT * FROM user WHERE id IN (1,2,3)

List<User> selectByIds(List<Integer> ids);

<select id="selectByIds" resultType="User">
    SELECT * FROM user
    <where>
        <foreach collection="list"
                 item="id"
                 open="id IN ("
                 separator=","
                 close=")">
            #{id}
        </foreach>
    </where>
</select>

SELECT * FROM user
WHERE id IN (1,2,3)

switch-case

<select id="select" resultType="User">
    SELECT * FROM user
    <where>
        <choose>
            <when test="username != null">
                username = #{username}
            </when>
            <when test="age != null">
                age = #{age}
            </when>
            <otherwise>
                gender = '男'
            </otherwise>
        </choose>
    </where>
</select>

<trim prefix="WHERE" prefixOverrides="AND |OR ">
    <if test="username != null">
        AND username = #{username}
    </if>
    <if test="age != null">
        AND age = #{age}
    </if>
</trim>

<where>

<trim>

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WHERE 1=1

age = 20,

List<User> selectCondition(User user);

<select id="selectCondition" resultType="User">
    SELECT * FROM user
    <where>
        <if test="username != null and username != ''">
            AND username LIKE CONCAT('%',#{username},'%')
        </if>
        <if test="age != null">
            AND age = #{age}
        </if>
        <if test="gender != null">
            AND gender = #{gender}
        </if>
    </where>
</select>

User user = new User();
user.setUsername("o");
user.setGender("男");
List<User> users = mapper.selectCondition(user);
users.forEach(System.out::println);

SELECT * FROM user
WHERE username LIKE '%o%'
AND gender = '男'

WHERE id = ?

ORDER BY age