The DISTINCT keyword is used to remove duplicate values from a result of a SQL or SPARQL query.
Use this tag only for questions about UNION, a keyword of the SQL language for combining the results of multiple SQL queries
Slower de-dupping tables
"Union distinct is slower than union all but you may need it for de-dupping."
from question "How to optimize select sum() on mysql?"
"Keep in mind that union will return a distinct list - duplicates will be removed but it will perform slower than using union all which will not remove duplicates"
from question "Checking if a string is found in one of multiple columns in mySQL"
"However the tables are huge and the union all i read its faster than distinct takes forever to execute even with just two tables let alone 6"
from question "MySQL: Optimizing SELECT from 6 huge identical tables with different data split up by timestamp"
"Union all is generally faster than using distinct or grouping"
from question "SQL Server : shortest syntax using distinct?"
Note rows outer
"A side note since you want all rows using union all is faster since it does not need to perform a distinct to eliminate duplicates"
from question "SQL select from table and get table name"
"I am currently using union all and a distinct in the outer query as this proved much faster than using union s for the uniqueness of data"
from question "Should I use a T-SQL function, view, or stored proc?"