MyBatis动态SQL：OGNL驱动的SQL组装艺术

StringBuilder sql = new StringBuilder("SELECT * FROM users WHERE 1=1");
List<Object> params = new ArrayList<>();
if (username != null && !username.isEmpty()) {
    sql.append(" AND username LIKE ?");
    params.add("%" + username + "%");
}
if (minAge != null) {
    sql.append(" AND age >= ?");
    params.add(minAge);
}
if (maxAge != null) {
    sql.append(" AND age <= ?");
    params.add(maxAge);
}
if (gender != null) {
    sql.append(" AND gender = ?");
    params.add(gender);
}
// 还要处理排序、分页...

<select id="findActiveBlogLike" resultType="Blog">
    SELECT * FROM BLOG
    WHERE state = 'ACTIVE'
    <if test="title != null">
        AND title like #{title}
    </if>
    <if test="author != null and author.name != null">
        AND author_name like #{author.name}
    </if>
</select>

<select id="findActiveBlogLike" resultType="Blog">
    SELECT * FROM BLOG
    WHERE state = 'ACTIVE'
    <choose>
        <when test="title != null">
            AND title like #{title}
        </when>
        <when test="author != null and author.name != null">
            AND author_name like #{author.name}
        </when>
        <otherwise>
            AND featured = 1
        </otherwise>
    </choose>
</select>

<select id="findBlog" resultType="Blog">
    SELECT * FROM BLOG
    WHERE
    <if test="state != null">
        state = #{state}
    </if>
    <if test="title != null">
        AND title like #{title}
    </if>
</select>

<select id="findBlog" resultType="Blog">
    SELECT * FROM BLOG
    <where>
        <if test="state != null">
            state = #{state}
        </if>
        <if test="title != null">
            AND title like #{title}
        </if>
    </where>
</select>

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

<trim prefix="WHERE" prefixOverrides="AND |OR ">
    ...
</trim>

<select id="findByIds" resultType="User">
    SELECT * FROM users
    WHERE id IN
    <foreach collection="ids" item="id" open="(" separator="," close=")">
        #{id}
    </foreach>
</select>

<insert id="batchInsert">
    INSERT INTO users (username, email, age) VALUES
    <foreach collection="list" item="user" separator=",">
        (#{user.username}, #{user.email}, #{user.age})
    </foreach>
</insert>

<select id="findByUsername" resultType="User">
    <bind name="pattern" value="'%' + username + '%'"/>
    SELECT * FROM users WHERE username LIKE #{pattern}
</select>

<!-- 定义字段列表 -->
<sql id="userColumns">
    id, username, email, age, create_time
</sql>
<!-- 定义通用的 WHERE 条件 -->
<sql id="userWhere">
    <where>
        <if test="username != null">
            AND username LIKE CONCAT('%', #{username}, '%')
        </if>
        <if test="email != null">
            AND email = #{email}
        </if>
    </where>
</sql>
<!-- 使用 -->
<select id="findAll" resultType="User">
    SELECT <include refid="userColumns"/>
    FROM users
</select>
<select id="findByCondition" resultType="User">
    SELECT <include refid="userColumns"/>
    FROM users
    <include refid="userWhere"/>
</select>

// DynamicSqlSource 的核心逻辑（简化）
public BoundSql getBoundSql(Object parameterObject) {
    DynamicContext context = new DynamicContext(configuration, parameterObject);
    rootSqlNode.apply(context);  // 遍历 SqlNode 树，动态生成 SQL
    // 解析 SQL 中的 #{}
    return sqlSourceParser.parse(context.getSql()).getBoundSql(parameterObject);
}

package com.example.demo.mapper;
import com.example.demo.entity.User;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Param;
import java.util.List;
@Mapper
public interface UserMapper {
    /**
     * 多条件查询用户列表
     * @param username 用户名（模糊匹配）
     * @param gender 性别
     * @param minAge 最小年龄
     * @param maxAge 最大年龄
     */
    List<User> findUsers(@Param("username") String username,
                         @Param("gender") String gender,
                         @Param("minAge") Integer minAge,
                         @Param("maxAge") Integer maxAge);
}

<?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.example.demo.mapper.UserMapper">
    <select id="findUsers" resultType="com.example.demo.entity.User">
        SELECT id, username, email, gender, age, create_time
        FROM users
        <where>
            <if test="username != null and username != ''">
                AND username LIKE CONCAT('%', #{username}, '%')
            </if>
            <if test="gender != null and gender != ''">
                AND gender = #{gender}
            </if>
            <if test="minAge != null">
                AND age >= #{minAge}
            </if>
            <if test="maxAge != null">
                AND age <= #{maxAge}
            </if>
        </where>
        ORDER BY create_time DESC
    </select>
</mapper>

void deleteByIds(@Param("ids") List<Integer> ids);

<delete id="deleteByIds">
    DELETE FROM users
    WHERE id IN
    <foreach collection="ids" item="id" open="(" separator="," close=")">
        #{id}
    </foreach>
</delete>

DELETE FROM users WHERE id IN (1, 2, 3, 5, 8)