1. 概述

　　MyBatis中动态SQL包括元素：

2. if元素

　　if元素是最常用的判断语句，常与test属性联合使用。

2.1 if

<resultMap id="baseResultMap" type="com.libing.helloworld.model.Role">
    <id property="id" column="id" />
    <result property="roleName" column="role_name" />
</resultMap>

<select id="findBySearchText" resultMap="baseResultMap">
    SELECT
        id,
        role_name
    FROM
        role
    WHERE 1 = 1
    <if test="searchText != null and searchText != ‘‘">
        AND role_name LIKE CONCAT(‘%‘, #{searchText,jdbcType=VARCHAR}, ‘%‘)
    </if>
    ORDER BY id ASC
</select>

2.2 if + where

<select id="findBySearchText" resultMap="baseResultMap">
    SELECT
        id,
        role_name
    FROM
        role
    <where>
        <if test="id > 0">
            id >= #{id}
        </if>
        <if test="searchText != null and searchText != ‘‘">
            AND role_name LIKE CONCAT(CONCAT(‘%‘,#{searchText,jdbcType=VARCHAR}),‘%‘)
        </if>
    </where>
    ORDER BY id ASC
</select>

　　MyBatis中where标签会判断如果所包含的标签中有返回值，则插入一个‘where’。此外，如果标签返回的内容是以AND或OR开头，则自动删除开头的AND或OR。

2.3 if + set

<update id="update" parameterType="com.libing.helloworld.model.Role">
    UPDATE role
    <set>
        <if test="roleName != null and roleName != ‘‘">
            role_name = #{roleName},
        </if>
        <if test="remark != null and remark != ‘‘">
            remark LIKE CONCAT(‘%‘, #{remark, jdbcType=VARCHAR}, ‘%‘)
        </if>
    </set>
    WHERE id = #{id}
</update>

　　上面形式，当ramark=null时，动态SQL语句会由于多出一个“,”而错误。

3. choose（when,otherwise）元素

<select id="findByCondition" resultMap="baseResultMap">
    SELECT
        id,
        role_name
    FROM
        role
    <where>
        <choose>
            <when test="id > 0">
                id >= #{id}
            </when>
            <otherwise>
                AND role_name LIKE CONCAT(‘%‘, #{roleName, jdbcType=VARCHAR}, ‘%‘)
            </otherwise>
        </choose>
    </where>
    ORDER BY id ASC
</select>

4.trim元素

4.1 trim：if + where

<select id="findByCondition" resultMap="baseResultMap">
    SELECT
        id,
        role_name
    FROM
        role
    <trim prefix="where" prefixOverrides="AND | OR">
        <if test="id > 0">
            id >= #{id}
        </if>
        <if test="roleName != null and roleName != ‘‘">
            AND role_name = LIKE CONCAT(‘%‘, #{roleName, jdbcType=VARCHAR}, ‘%‘)
        </if>
    </trim>
    ORDER BY id ASC
</select>

4.2 trim：if + set

<update id="update" parameterType="com.libing.helloworld.model.Role">
    UPDATE role
    <trim prefix="set" suffixOverrides=",">
        <if test="roleName != null and roleName != ‘‘">
            role_name = #{roleName},
        </if>
        <if test="remark != null and remark != ‘‘">
            remark LIKE CONCAT(‘%‘, #{remark, jdbcType=VARCHAR}, ‘%‘)
        </if>
    </trim>
    WHERE id = #{id}
</update>

5. foreach元素