Re-learning database integration-use of Union and union and

UnionAndAndUse

Union The purpose of the command is SQL Statement results are merged. From this perspective, Union And Join Some of them are similar, because both commands can retrieve data from multiple tables. Union One limit is two. SQL The field generated by the statement must be of the same data type. In addition, when we use Union In this command, we will only see different data values ( Similar Select distinct) .  

Union all The purpose of this command is SQL Statement results are merged. Union all And Union The difference is that Union all Each qualified item is listed, regardless of whether the item value is repeated.

For example

Select * from (select * From fa_family where fa_name = '1' Union select * From fa_family where fa_name = '1') as fa_family