What is the difference between UNION and UNION ALL?

Question

What is the difference between UNION and UNION ALL?

Answer 1

UNION removes duplicate records in other hand UNION ALL does not. But one need to check the bulk of data that is going to be processed and the column and data type must be same.

since union internally uses "distinct" behavior to select the rows hence it is more costly in terms of time and performance. like

select project_id from t_project
union
select project_id from t_project_contact  

this gives me 2020 records

on other hand

select project_id from t_project
union all
select project_id from t_project_contact

gives me more than 17402 rows

on precedence perspective both has same precedence.