in是把外表和内表作hash连接,而exists是对外表作loop循环,每次loop循环再对内表进行查询,一直以来认为exists比in效率高的说法是不准确的。 如果查询的两个表大小相当,那么用in和exists差别不大;如果两个表中一个较小一个较大,则子查询表大的用exists,子查询表小的用in; 例如:表A(小表),表B(大表) select ...
CREATE PROCEDURE CheckTableAndColumn AS BEGIN IF EXISTS (SELECT * FROM information_schema.tables WHERE table_name = 'your_table') BEGIN IF EXISTS (SELECT * FROM information_schema.columns WHERE table_name = 'your_table' AND column_name = 'your_column') BEGIN PRINT 'Table and column exist....
IF OBJECT_ID('HR.Employees') IS NULL--this object does exist in the sample databaseBEGINPRINT'The specified object does not exist';ENDELSEBEGINPRINT'The specified object exists';END; 在資料操作作業中,搭配 EXISTS 關鍵字使用 IF 是有效率地檢查是否存在的實用工具,如下列...
Here are 4 examples illustrating when you would use IF EXISTS and when you would use IF NOT ...
if maps exist if more sustainable if must walk if my departure could if my dreams get real if need different ser if never i met you if no if not cencerned with if not clear if not for me if not if not light if not set if not the objective if nothing goes wrong if one day you...
Here are 4 examples illustrating when you would use IF EXISTS and when you would use IF NOT ...
DROPTABLEexistdemo; The output of the execution of the above query is as shown below – It raises an error saying this table is unknown as it does not exist on the educba database. Let us try using the IF EXISTS clause in the same query statement – ...
if everyone shared an if everything works o if exist if forwarded if god exists if happy little blue if have an accident if he asks if youre a if he comes bck if he hath wronged th if he kept not if he wanted if heaven and hell de if his offering a bur if history holds if i...
USEtestdb;IFEXISTS(SELECT1FROMinformation_schema.TABLESWHERETABLE_SCHEMA='testdb'ANDTABLE_NAME='employees')THENDROPTABLEemployees;SELECT'表已删除。';ELSESELECT'表不存在。';ENDIF; 1. 2. 3. 4. 5. 6. 7. 8. 总结 在MySQL中,我们可以使用IF EXISTS THEN语法来检查表或数据库是否存在,并根据结果执...
If it weren't for Rachel Carson, the environmental movement might not exist today. 2016年高考英语全国卷1 阅读理解 阅读A 原文 If it's daytime in your destination, try to stay awake. 2015年高考英语重庆卷 完形填空 B 原文 If Louis could find his way out of his problems, why should I ever...