[위로]

<원인>

<SQL문>

select * From dbo.authors where au_lname like 'white%'
Union
select * From dbo.authors where au_lname like 'Dul%'


select * From dbo.authors where au_lname like 'white%'
Union all
select * From dbo.authors where au_lname like 'Dul%'

-- 유니크한1개의 값을 Return
select * From dbo.authors where au_lname like 'white%'
Union
select * From dbo.authors where au_lname like 'white%'

-- 중복된 2개의 값을 Return
select * From dbo.authors where au_lname like 'white%'
Union all
select * From dbo.authors where au_lname like 'white%'

<조치 사항>

  1. Union 과 Union All을 구분하여 사용

<기타>

  1. Union All 을 기본적으로 사용