SQL set operation

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Catalog

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The addition and subtraction of tables

What is set operation

Addition of tables —— UNION

Precautions for set operation

Select the common part of the table —— INTERSECT

Recorded subtraction —— EXCEPT

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The addition and subtraction of tables

• What is set operation

  • Sets represent... In the field of mathematics “（ All kinds of ） The sum of things ”, Represents a collection of records in the database domain .

• Addition of tables —— UNION

• surface 1
• 

• surface 2

• 

    You can see the watch 1 Of 004、5、7、8 In the table 2 There is no such thing as , At the same time in the table 2 Medium 9、10 In the table 1 It doesn't exist in the world

  SELECT product_id, product_name
  FROM Product
  UNION
  SELECT product_id, product_name
  FROM Product2;

   

  You can see union Is to find the union , And the weight is automatically removed , The set operator removes duplicate records .

  

    If we want to keep duplicate lines, we just need to Add ALL that will do

  

    You can see that the repeated lines have been saved

• Precautions for set operation

  • The number of columns of the record as the operands must be the same
  • The type of the column in the record as the operation object must be consistent
  • You can use any SELECT sentence , but ORDER BY Clause can only be used last time

• Select the common part of the table —— INTERSECT

  • Select the common part of the two record sets INTERSECT （ intersection ）

  • SELECT product_id, product_name
    FROM Product
    INTERSECT
    SELECT product_id, product_name
    FROM Product2
    ORDER BY product_id;

    

     

      You can see INSTERSECT Is to find the intersection of two records (PS:MYSQL Not available in INSTERSECT）

Recorded subtraction —— EXCEPT

• Subtraction EXCEPT （ Difference set ）（PS: Only Oracle Don't use EXCEPT , and
  Is to use its unique MINUS Operator . Use Oracle Users of , Please use MINUS
  Instead of EXCEPT . Besides ,MySQL Not yet EXCEPT , So it can't be used .)

• SELECT product_id, product_name
  FROM Product
  EXCEPT
  SELECT product_id, product_name
  FROM Product2
  ORDER BY product_id;