Question

当我运行这两个查询时，SQL Server Management Studio中的客户端统计信息显示第二个查询的总执行时间更快。谁能告诉我为什么第二个查询更快？

SELECT MERCHANT_NO,
(SELECT CASE WHEN CITY IS NULL OR CITY='' THEN m.CITY  
 ELSE 
    COALESCE((SELECT c.CITY
     FROM CODE_CITY c
     WHERE m.CITY=c.CODE_CITY_ID
    ), m.CITY)    
  END) AS CITY,
(SELECT CASE WHEN STATE IS NULL OR STATE='' THEN m.STATE  
 ELSE 
    COALESCE((SELECT s.STATE
     FROM CODE_STATE s
     WHERE m.STATE=s.CODE_STATE_ID
    ), m.STATE)    
  END) AS STATE,
(SELECT MERCHANT_CAT_DESC FROM MERCHANT_CAT mc WHERE mc.MERCHANT_CAT_ID=m.MERCHANT_CAT_ID) AS MERCHANT_CAT_DESC 
FROM MERCHANT m 
WHERE 1=1

SELECT MERCHANT_NO, 
(SELECT CASE WHEN t.CITY IS NULL OR t.CITY='' THEN m.CITY 
  ELSE 
     t.CITY 
  END AS CITY
 FROM(
      SELECT 
       CASE WHEN CITY IS NULL OR CITY='' THEN '' 
          ELSE 
             (SELECT c.CITY
               FROM CODE_CITY c
               WHERE merc.CITY=c.CODE_CITY_ID
              )      
          END AS CITY
          FROM MERCHANT merc
          WHERE merc.MERCHANT_NO=m.MERCHANT_NO
) t ) AS CITY_DESC,  
(SELECT CASE WHEN cs.STATE IS NULL OR cs.STATE='' THEN m.STATE 
  ELSE 
     cs.STATE 
  END AS STATE
 FROM(
      SELECT 
       CASE WHEN STATE IS NULL OR STATE='' THEN '' 
          ELSE 
             (SELECT s.STATE
               FROM CODE_STATE s
               WHERE merc.STATE=s.CODE_STATE_ID
              )      
          END AS STATE
          FROM MERCHANT merc
          WHERE merc.MERCHANT_NO=m.MERCHANT_NO
) cs ) AS STATE_DESC, 
(SELECT MERCHANT_CAT_DESC FROM MERCHANT_CAT mc WHERE mc.MERCHANT_CAT_ID=m.MERCHANT_CAT_ID) AS MERCHANT_CAT_DESC 
FROM MERCHANT m 
WHERE 1=1

Answer 1

您在第二次查询中未使用 COALESCE 数据。所以只有它比第一次查询更快。

哪个SQL查询执行得更快

1 个答案: