How to use the dynamic SQL tags in MyBatis?

MyBatis’s dynamic SQL tags allow us to dynamically generate different SQL fragments based on conditions in our SQL statements, enabling more flexible queries.

Here are some examples of using MyBatis dynamic SQL tags:

1. if tag: generate SQL fragments based on conditions.

<select id="selectUsers" resultType="User">
    SELECT id, username, email
    FROM users
    <where>
        <if test="username != null">
            AND username = #{username}
        </if>
        <if test="email != null">
            AND email = #{email}
        </if>
    </where>
</select>

1. select SQL snippet based on condition进行选择不同的SQL片段

<select id="selectUsers" resultType="User">
    SELECT id, username, email
    FROM users
    <where>
        <choose>
            <when test="username != null">
                AND username = #{username}
            </when>
            <when test="email != null">
                AND email = #{email}
            </when>
            <otherwise>
                AND id > 0
            </otherwise>
        </choose>
    </where>
</select>

1. “foreach tag: used to iterate through a collection to build SQL fragments”

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

These are the commonly used dynamic SQL tags in MyBatis, which can easily facilitate flexible SQL queries. For more information on how to use MyBatis dynamic SQL tags, please refer to the official documentation.