How to handle null values in MyBatis?

There are several ways to handle null values in MyBatis, depending on the specific functionality and business logic you want to achieve.

1. Use the if tag for making a decision:

<select id="selectUsers" parameterType="java.util.Map" resultType="User">
    SELECT * FROM users
    <where>
        <if test="username != null">
            AND username = #{username}
        </if>
        <if test="age != null">
            AND age = #{age}
        </if>
    </where>
</select>

1. Conditional selection using choose and when tags.

<select id="selectUsers" parameterType="java.util.Map" resultType="User">
    SELECT * FROM users
    <where>
        <choose>
            <when test="username != null">
                AND username = #{username}
            </when>
            <when test="age != null">
                AND age = #{age}
            </when>
        </choose>
    </where>
</select>

1. Define common validation logic using the SQL tag.

<sql id="condition">
    <if test="username != null">
        AND username = #{username}
    </if>
    <if test="age != null">
        AND age = #{age}
    </if>
</sql>

<select id="selectUsers" parameterType="java.util.Map" resultType="User">
    SELECT * FROM users
    <where>
        <include refid="condition" />
    </where>
</select>

By using the above methods, you can choose the appropriate way to handle null values as needed, making SQL statements more flexible and readable.