SQL query result merging union all usage _ database skills

Copy codeThe Code is as follows:
-- Merge duplicate rows
Select * from
Union
Select * from B


-- Duplicate rows are not merged.
Select * from
Union all
Select * from B


Sort by a Field
-- Merge duplicate rows
Select *
From (
Select * from
Union
Select * from B) AS T
Order by field name

-- Duplicate rows are not merged.
Select *
From (
Select * from
Union all
Select * from B) AS T
Order by field name

// SQL server
Select * From (
Select top 2 id, adddate, title, url from bArticle where ClassId = 1 order by adddate desc)
Union All
Select * From (
Select top 2 id, adddate, title, url from bArticle where ClassId = 2 order by adddate desc) B
Union All
Select * From (
Select top 2 id, adddate, title, url from bArticle where ClassId = 3 order by adddate desc) C
Union All
Select * From (
Select top 2 id, adddate, title, url from bArticle where ClassId = 4 order by adddate desc) D


// Mysql
Select * From (
Select id, adddate, title, url from bArticle where ClassId = 1 order by adddate desc limit 0, 2)
Union All
Select * From (
Select id, adddate, title, url from bArticle where ClassId = 2 order by adddate desc limit 0, 2) B
Union All
Select * From (
Select id, adddate, title, url from bArticle where ClassId = 3 order by adddate desc limit 0, 2) C
Union All
Select * From (
Select id, adddate, title, url from bArticle where ClassId = 4 order by adddate desc limit 0, 2) D