17、mybatis-动态sql-f choose、when、o

一、choose(when，otherwise)

1、说明

有时候我们并不想应用所有的条件，而只是想从多个选项中选择一个。MyBatis提供了choose 元素，按顺序判断when中的条件出否成立，如果有一个成立，则choose结束。当choose中所有when的条件都不满则时，则执行 otherwise中的sql。类似于Java 的switch 语句，choose为switch，when为case，otherwise则为default

2、语法结构

<select..>
SQL语句1
<choose>
<when test="条件表达式">
    SQL语句2
</when>
<otherwise>
    SQL语句3
</otherwise>
</choose>
</select>

3、栗子

    <resultMap id="BaseResultMap" type="User">
        <id column="uid" jdbcType="INTEGER" property="uid" />
        <result column="name" jdbcType="VARCHAR" property="name" />
        <result column="password" jdbcType="VARCHAR" property="password" />
        <result column="phone" jdbcType="VARCHAR" property="phone" />
        <result column="email" jdbcType="VARCHAR" property="email" />
        <result column="join_date" jdbcType="TIMESTAMP" property="joinDate" />
        <result column="login_date" jdbcType="TIMESTAMP" property="loginDate" />
        <result column="status" jdbcType="TINYINT" property="status" />
    </resultMap>
    <sql id="Base_Column_List">
        `uid`, `name`, `password`, phone, email, join_date, login_date, `status`
    </sql>
    <select id="findUserChoose" resultMap="BaseResultMap">
        SELECT
        <include refid="Base_Column_List"/>
        FROM user u
        <where>
            <choose>
                <when test="user.name != null and user.name != '' ">
                    u.name LIKE CONCAT(CONCAT('%', #{user.name}),'%')
                </when>
                <when test="user.phone != null and user.phone != '' ">
                    AND u.phone = #{user.phone}
                </when>
                <when test="user.email != null and user.email != ''">
                    AND u.email = #{user.name,jdbcType=VARCHAR}
                </when>
                <otherwise>
                </otherwise>
            </choose>
        </where>
    </select>