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  • Except/Intersect/Union

    1. Union

    Syntax:

    -- union 10 records
    select * from Table1
    union
    select * from Table2
    --
    select EnglishProductName from Table1
    union
    select EnglishProductName from Table2

    Analysis:

    You can union more than two tables --- simply add more Union operators and Select statements. A union is not the same as a join. A join puts the tables together horizontally, usually based on a primary-to-foreign-key relationship --- it’s for creating a single result from often dissimilar tables (denormalization). A union appends tables; is puts them together vertically --- it’s for creating a single result from similar tables.

    2. Union All

    Syntax:

    -- union all 11 records
    select * from Table1
    union all
    select * from Table2

    Analysis:

    The record that is common to both tables. The union operator suppresses duplicates, while union all does not.

    3. Intersect

    Syntax:

    -- intersect 1 record
    select * from Table1
    intersect
    select * from Table2

    Analysis:

    Intersect returns the common record from the two tables. It shows just one copy of the record, not both of them. This is a simple, yet powerful, set operation to see if you have the same data in more than one table.

    4. Except

    Syntax:

    -- except 8 records
    select * from Table1
    except
    select * from Table2

    Analysis:

    Except is showing all of the records from Table1 that do not also appear in Table2. Please not that Table1 is the first table in the query.

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  • 原文地址:https://www.cnblogs.com/aot/p/3148546.html
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