*、select 1 from table where.....
The syntax is similar to select * from table where..... But in terms of efficiency: 1 > table_col > *, because there is no need to query the data dictionary The total number of query data: sum(1) is equivalent to sum(*), but the actual operating efficiency remains to be verified~
*. There will be such a demand in many times: the time and space information should be ranked in the reverse order of paging query, but the mechanism of oracle is that null is the largest, so:
The solution is: since null is the largest, then set the null value to a default minimum value when sorting, tools such as case when variable then result 1 else result 2 end (this method can also be used with hibernate-hql)