记录一个SQL语句 case select 1

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select Code, CodeName, CodeAlias, ComCode, OtherSign   from ldcode  where codetype = 'edorapptype'    and code <> (case          when (select 1                  from lccont b                 where b.contno = 'W86190001290019'                   and b.trdingchannel like 'ZFB%') = 1 then           '-9'          else           '9'        end)    and othersign = '1'  order by length(code), code asc

　　去掉(case when)

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select Code, CodeName, CodeAlias, ComCode, OtherSign  from ldcode where codetype = 'edorapptype'   and code <> '-9' and othersign = '1'

　　分析case when  1= 1 then '-9' else '9' end

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(case          when (select 1                  from lccont b                 where b.contno = 'W86190001290019'                   and b.trdingchannel like 'ZFB%') = 1 then           '-9'          else           '9'        end)

　　分析 select 1

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select 1  from lccont b                 where b.contno = 'W86190001290019'                   and b.trdingchannel like 'ZFB%'

　　select * ,select 某字段,

可以用来查询表中是否有符合条件的记录(比如select 1 from seckill where id = 1001;)，select 1一般用来当作条件使用，比如exists( select 1 from 表名)等。select 1的效率比select 列名和select*快，因为不用查字典表。