SELECT '遠大' 企業名稱,
       sgpro.project_name 工程名稱,
       pur.product_type 產品類別,
       pur.product_name 產品名稱,
       pur.purchase_num 涉及數量,
       pur.product_regdate 材料進場時間,
       sgpro.build_licence 施工許可證號,
       sgpro.contractor_name 總包單位名稱
  FROM t_purchase_info pur, t_allproject_info sgpro
 WHERE pur.project_id = sgpro.id
   AND pur.id in (select distinct regexp_substr(ids, '[^,]+', 1, level) project_id
                    from (select dbms_lob.substr(project_ids) ids
                            from g_enterprise_score
                           where class in (3, 4)
                             and score_type = 5
                             AND bad_no = 1
                             AND enterprise_id in
                                 (select distinct id
                                    from bemms.g_enterprise_info
                                   where name like '%遠大%')
                             and project_ids is not null)
                  connect by level <=
                             length(dbms_lob.substr(ids)) -
                             length(replace(dbms_lob.substr(ids), ',', '')) + 1
                  )
 

其中，將clob轉化為varchar，利用dbms_lob.substr，例：

select dbms_lob.substr(project_ids)
  from g_enterprise_score
 where class in (3, 4)
   and score_type = 5
   AND bad_no = 1
   AND enterprise_id in (select distinct id
                           from bemms.g_enterprise_info
                          where name like '%遠大%')
   and project_ids is not null ;
 

將一行字串（逗號分割）轉化多行，利用regexp_substr() connect by level，例：

select distinct regexp_substr(ids, '[^,]+', 1, level) project_id
  from (select dbms_lob.substr(project_ids) ids
          from g_enterprise_score
         where class in (3, 4)
           and score_type = 5
           AND bad_no = 1
           AND enterprise_id in (select distinct id
                                   from bemms.g_enterprise_info
                                  where name like '%遠大%')
           and project_ids is not null)
connect by level <= length(dbms_lob.substr(ids)) -
           length(replace(dbms_lob.substr(ids), ',', '')) + 1
 

以上SQL執行效率特別低，我們歡歡大神說過： 越是高階的函式越有可能影響效率，還是最基礎的函式好，效率槓槓的。

其中儘量少用in，用exists，或者instr，經過優化後的SQL如下：

SELECT '北京XXX股份有限公司' 企業名稱,
       sgpro.project_name 工程名稱,
       pur.product_type 產品類別,
       pur.product_name 產品名稱,
       pur.purchase_num 涉及數量,
       pur.product_regdate 材料進場時間,
       sgpro.build_licence 施工許可證號,
       sgpro.contractor_name 總包單位名稱
  FROM t_purchase_info pur, t_allproject_info sgpro
 WHERE pur.project_id = sgpro.id
   AND instr((select ',' || wm_concat(dbms_lob.substr(g.project_ids)) || ',' ids
               from g_enterprise_score g
              where exists (select id
                       from bemms.g_enterprise_info e
                      where g.enterprise_id = e.id
                        and name like '%北京%')
                and (g.class = 3 or g.class = 4)
                and g.score_type = 5
                AND g.bad_no = 1
                and project_ids is not null),
             ',' || pur.id || ',') > 0