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2025-04-03 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > Database >
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Shulou(Shulou.com)06/01 Report--
Set operation
UNION and UNION ALL
UNION: column name. If the data type is consistent, the result will be sorted automatically by default ASC.
UNION ALL: the result will not be duplicated or sorted, and everything else is the same as UNION.
Fill in missing columns with things like to_char (null), to_number (null), to_date (null)
Such as:
Select job_id
From employees
UNION
Select job_id
From retired_employees
Such as:
Select job_id,department_id
From employees
UNION ALL
Select job_id,department_id
From retired_employees
Order by job_id
Intersect take intersection
The results will be sorted and duplicated.
Such as:
Select manager_id,department_id
From employees
INTERSECT
Select manager_id,department_id
From retired_employees
Pre-MINUS select results minus and post-select de-repeating results
Such as:
Select employee_id, job_id
From employees
Where department_id=80
MINUS
Select employee_id, job_id
From retired_employees
Where department_id=90
The above four outputs are only concerned with the definition of select in Article 1.
Order by can only be placed at the end, and can only appear once, and cannot be placed in a subquery.
Parenthes parentheses can change the execution priority.
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