T-SQL grammar learning one (continuous update)

T-SQL grammar learning (1)

Section 1 Uncommon Statements

Uncommon statements - refer to some uncommon query statements, not for business data queries

• SET STATISTICS IO ON (used to query the number of logical reads, the number of physical reads)
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• select @@version (query the database version of the current instance)

Section 2 Precautions for Operating Database Management Tools

When we are using the database graphical management interface, if we forget to add conditions when performing update, insert, delete operations, then a very serious problem will occur, the data of the entire table, and even the entire database may be deleted by you So when doing these dangerous operations, I recommend adding transactions, as follows:

  begin tran hcx  --创建并命名事务
  update MCSMCB
  set MCB004=c.MYA013,MCB021=c.MYA003
  from
   MCSMCB b
   left join MCSMCA a on b.MCB001=a.MCA001
   left join EB_DuoMoCh.dbo.BASMYA c on c.MYA001=a.MCA002 and  RTRIM(LTRIM(c.MYA004))=b.MCB003 AND b.MCB015=c.MYA002
  
    --先使用select将要update的列，以及要update的值查询出来，做好对比，然后再执行update操作，并select出来，最后commit
   select MCB001,MCA002,c.MYA004,b.MCB003,c.MYA013,b.MCB004,c.MYA003,b.MCB021,b.MCB015,c.MYA002 from
   MCSMCB b
   left join MCSMCA a on b.MCB001=a.MCA001
   left join EB_DuoMoCh.dbo.BASMYA c on c.MYA001=a.MCA002 and  RTRIM(LTRIM(c.MYA004))=b.MCB003 AND b.MCB015=c.MYA002
  
  rollback tran hcx  --回滚事务
  COMMIT TRAN hcx