oralce 笔记

　　查某一表的行数

select max(rownum) from tablename

　　插入数据之前判断是否重复

insert into tablename (coloum1,coloum2) select coloum1,coloum2  from dual where not exists( select 1 from tablename where coloum1 ="value1" and coloum2="value2";

　　查询时检索数据库中字段

select coloum1 , coloum1 ,coloum3 from  table   where   instr('coloum1',查询字符)>0 or instr('coloum3',查询字符)>0 or  instr('coloum3',查询字符)>0

　　去除表中有字段为空的数据

delete  from  table  where coloum is null ;