4、動態 SQL、SQL 語句構建器（筆記）

一、if

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG
  <where>
    <if test="state != null">
         state = #{state}
    </if>
    <if test="title != null">
        AND title like #{title}
    </if>
    <if test="author != null and author.name != null">
        AND author_name like #{author.name}
    </if>
  </where>
</select>

or
<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG
  where 1=1
    <if test="state != null">
         state = #{state}
    </if>
    <if test="title != null">
        AND title like #{title}
    </if>
    <if test="author != null and author.name != null">
        AND author_name like #{author.name}
    </if>
</select>

二、choose、when、otherwise

<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>
  </choose>
</select>

三、trim、set
<trim prefix="" suffix="" suffixOverrides="" prefixOverrides=""></trim>
prefix： 添加前綴
suffix: 添加後綴
suffixOverrides： 去除前綴
prefixOverrides： 去除後綴

<insert id="insert" parameterType="com.tortuousroad.groupon.cart.entity.Cart">
        insert into cart
        <trim prefix="(" suffix=")" suffixOverrides=",">
            <if test="id != null">
                id,
            </if>
            <if test="userId != null">
                user_id,
            </if>
            <if test="dealId != null">
                deal_id,
            </if>
            <if test="dealSkuId != null">
                deal_sku_id,
            </if>
         
        </trim>
        <trim prefix="values (" suffix=")" suffixOverrides=",">
            <if test="id != null">
                #{id,jdbcType=BIGINT},
            </if>
            <if test="userId != null">
                #{userId,jdbcType=BIGINT},
            </if>
            <if test="dealId != null">
                #{dealId,jdbcType=BIGINT},
            </if>
            <if test="dealSkuId != null">
                #{dealSkuId,jdbcType=BIGINT},
            </if>
           
        </trim>
    </insert>

set

<update id="updateAuthorIfNecessary">
  update Author
    <set>
      <if test="username != null">username=#{username},</if>
      <if test="password != null">password=#{password},</if>
      <if test="email != null">email=#{email},</if>
      <if test="bio != null">bio=#{bio}</if>
    </set>
  where id=#{id}
</update>

四、foreach

<select id="selectPostIn" resultType="domain.blog.Post">
  SELECT *
  FROM POST P
  WHERE ID in
  <foreach item="item" index="index" collection="list"
      open="(" separator="," close=")">
        #{item}
  </foreach>
</select>

五、script
要在帶註解的映射器接口類中使用動態 SQL，能夠使用 script 元素。好比:

@Update({"<script>",
  "update Author",
  "  <set>",
  "    <if test='username != null'>username=#{username},</if>",
  "    <if test='password != null'>password=#{password},</if>",
  "    <if test='email != null'>email=#{email},</if>",
  "    <if test='bio != null'>bio=#{bio}</if>",
  "  </set>",
  "where id=#{id}",
  "</script>"})
void updateAuthorValues(Author author);

六、bind
bind 元素容許你在 OGNL 表達式之外建立一個變量，並將其綁定到當前的上下文。好比：

<select id="selectBlogsLike" resultType="Blog">
  <bind name="pattern" value="'%' + _parameter.getTitle() + '%'" />
  SELECT * FROM BLOG
  WHERE title LIKE #{pattern}
</select>

七、多數據庫支持
若是配置了 databaseIdProvider，你就能夠在動態代碼中使用名爲 「_databaseId」 的變量來爲不一樣的數據庫構建特定的語句。好比下面的例子：

<insert id="insert">
  <selectKey keyProperty="id" resultType="int" order="BEFORE">
    <if test="_databaseId == 'oracle'">
      select seq_users.nextval from dual
    </if>
    <if test="_databaseId == 'db2'">
      select nextval for seq_users from sysibm.sysdummy1"
    </if>
  </selectKey>
  insert into users values (#{id}, #{name})
</insert>

5、SQL 語句構建器