Interview Questions--database

1. The difference between the Union and UNION all

is to display the results of multiple slelect statements as a whole.

The difference is that union automatically compresses duplicate results in multiple result sets (the system automatically removes the duplicate tuples), and union ALL displays all results (preserving duplicate Ganso), whether or not duplicates.

Union is inefficient because it is being scanned for duplicate values. If the merge does not intentionally delete duplicate rows, then use UNION ALL

Two SQL statements to be federated, the number of fields must be the same, and the field type is "compatible" (consistent).

2. Paging

With RowNum.

Attention:

Select *  from    (select T.*fromorder by t.id) TableB  whereTableBbetween1andten

The above statement executes the ORDER BY clause after the rownum, so the sorting result is not correct at all.

Adjusted to

Select *  from    (Select A.*from      (Select*from)  orderbydesc) a   wherebetween1  andten

This is the right sort of order, but the SQL statement can be further optimized:

 select  *    ( select  a.*   from  --  (select  *  from   table_name) A  where  ROWNUM   )  where  RN >=  1  

Interview Questions--database