MyBatis limit分頁設置的實現

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錯誤的寫法：

<select id="queryMyApplicationRecord" parameterType="MyApplicationRequest" resultMap="myApplicationMap">
 SELECT
 a.*,
 FROM
 tb_user a
 WHERE 1=1
 <if test="ids != null and ids.size()!=0">
  AND a.id IN
  <foreach collection="ids" item="id" index="index"
     open="(" close=")" separator=",">
   #{id}
  </foreach>
 </if>
 <if test="statusList != null and statusList.size()!=0">
  AND a.status IN
  <foreach collection="statusList" item="status" index="index"
     open="(" close=")" separator=",">
   #{status}
  </foreach>
 </if>
 ORDER BY a.create_time desc
 LIMIT (#{pageNo}-1)*#{pageSize},#{pageSize}; // 錯誤
</select>

 在MyBatis中LIMIT之後的語句不允許的變量不允許進行算數運算，會報錯。

 正確的寫法一：

<select id="queryMyApplicationRecord" parameterType="MyApplicationRequest" resultMap="myApplicationMap">
 SELECT
 a.*,
 FROM
 tb_user a
 WHERE 1=1
 <if test="ids != null and ids.size()!=0">
  AND a.id IN
  <foreach collection="ids" item="id" index="index"
     open="(" close=")" separator=",">
   #{id}
  </foreach>
 </if>
 <if test="statusList != null and statusList.size()!=0">
  AND a.status IN
  <foreach collection="statusList" item="status" index="index"
     open="(" close=")" separator=",">
   #{status}
  </foreach>
 </if>
 ORDER BY a.create_time desc
 LIMIT ${(pageNo-1)*pageSize},${pageSize}; （正確）
</select>

 正確的寫法二：（推薦）

<select id="queryMyApplicationRecord" parameterType="MyApplicationRequest" resultMap="myApplicationMap">
 SELECT
 a.*,
 FROM
 tb_user a
 WHERE 1=1
 <if test="ids != null and ids.size()!=0">
  AND a.id IN
  <foreach collection="ids" item="id" index="index"
     open="(" close=")" separator=",">
   #{id}
  </foreach>
 </if>
 <if test="statusList != null and statusList.size()!=0">
  AND a.status IN
  <foreach collection="statusList" item="status" index="index"
     open="(" close=")" separator=",">
   #{status}
  </foreach>
 </if>
 ORDER BY a.create_time desc
 LIMIT #{offSet},#{limit}; （推薦，代碼層可控）
</select>

 分析：方法二的寫法，需要再請求參數中額外設置兩個get函數，如下： 

@Data
public class QueryParameterVO {
 
 private List<String> ids;
 
 private List<Integer> statusList;
 
 // 前端傳入的頁碼
 private int pageNo; // 從1開始
 
 // 每頁的條數
 private int pageSize;
 
 // 數據庫的偏移
 private int offSet;
 
 // 數據庫的大小限制
 private int limit;
 
 // 這裡重寫offSet和limit的get方法
 public int getOffSet() {
  return (pageNo-1)*pageSize;
 }
 
 public int getLimit() {
  return pageSize;
 }
}

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