MyBatis----D(动态SQL---if、choose、trim、foreach)

动态SQL支持:
if、choose (when, otherwise)、trim (where, set)、foreach  ……………………
if       
动态 SQL 通常要做的事情是根据条件包含 where 子句的一部分。比如：
                <select id="findActive" resultType="Blog">   对应到Mapper接口中的同名方法上
                             SELECT * FROM BLOG WHERE state = ‘ACTIVE’ 
                       <if test="title != null">   AND title like #{title}   </if>
                </select>

                 test属性中是一个OGNL逻辑表达式，如果返回结果为true则执行标签体，否则不执行标签体

                参数中的title属性如果不为空，则 SELECT * FROM BLOG WHERE state = ‘ACTIVE’ and AND title like #{title} 

                insertSelective定义中有很多if判断，根据对应的属性生成不同的SQL语句，动态插入获取的优势是default设置生效  
                updateByPrimaryKeySelective同样处理
  
choose, when, otherwise可以实现开关分支判断
        <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>当多有的when不成立时执行这个标签体
                    </choose>

        </select> 

                    注意：其中的语法格式为元素类型为 "choose" 的内容必须匹配 "(when*,otherwise?)"

                             这个标签体中只要一个条件判断成立，则立即退出标签.可以使用这个标签模拟出if/else结构


where用于生成对应的where关键字
                <select id="selectCondition" parameterType="map" resultType="UserBean">
                                select * from t_users where 1=1
                                <if test="username!=null">
                                            username=#{username}
                                </if>
                                <if test="password!=null">
                                            and password=#{password}
                                </if>
                                <if test="birth!=null">
                                            and birth=#{birth}
                                </if>
                </select>

                                 <where>标签体中没有生成任何查询条件，则不会生成where关键字;如果生成了SQL语句，同时SQL语句以and/or开头，则自动剔除多余的and/or关键字

            <select id="selectCondition" parameterType="map" resultType="UserBean">
                        select * from t_users
                <where>
                        <if test="username!=null">
                                    and username=#{username} 
                        </if>
                        <if test="password!=null">
                                    and password=#{password}
                        </if>
                        <if test="birth!=null">
                                    and birth=#{birth}
                        </if>
                </where>
            </select>

                             set和where类似，用于生成update语句中的set关键字，如果标签体为空，则不会生成set关键;如果标签体不为空，则会自动剔除所生成SQL语句末尾的多余的逗号


        <update id="updateByPrimaryKeySelective" parameterType="com.yan.entity.User">
                                update t_users
            <set>
                    <if test="username != null">
                            username = #{username,jdbcType=VARCHAR},
                    </if>
                    <if test="password != null">
                            password = #{password,jdbcType=VARCHAR},
                    </if>
            </set>
                            where id = #{id,jdbcType=BIGINT}
        </update>


trim和where/set类似
        <trim prefix="WHERE 自动添加的标签体生成的SQL语句的前缀"    prefixOverrides="AND |OR 需要自动剔除的SQL语句的前缀，不会区分大小写"  suffix=",,,,自动添加的后缀"   suffixOverrides=",自动去除的后缀">
           ... 
        </trim>

        替代<where>
            <trim prefix="where" prefixOverrides="AND | OR">
                <if test="username!=null">
                        and username=#{username} 
                </if>
                <if test="password!=null">
                        and password=#{password}
                </if>
            </trim>

        替代<set>
            <trim prefix="set" suffixOverrides=",">
                    <if test="username != null">
                            username = #{username,jdbcType=VARCHAR},
                    </if>
                    <if test="password != null">
                            password = #{password,jdbcType=VARCHAR},
                    </if>
            </trim>

foreach----对一个集合进行遍历，通常是在构建 IN 条件语句的时候
                <if test="ids!=null">
                        and id in
                        <foreach collection="ids在map中的集合值key,这是一个OGNL表达式，按照key从map中获取对应的集合" 

index="kk序号，从0开始"

                                item="pp循环控制变量，每次迭代访问一个元素，就会临时赋值给它"

                                open="(生成内容的开头，只添加一次" 

                                close=")生成内容的结束，只添加一次" separator=",每个元素之间的分隔符，末尾处不会添加">
                    </foreach>
                </if>

可以将任何可迭代对象（如 List、Set 等）、Map 对象或者数组对象传递给 foreach 作为集合参数。当使用可迭代对象或者数组时，index 是当前迭代的次数，item 的值是本次迭代获取的元素。当使用 Map 对象（或者 Map.Entry 对象的集合）时，index 是键，item 是值