What is SQL get connection

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Catalog

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coupling （ Join tables in columns ）

What is connection

Internal connection —— INNER JOIN

Extraneous junction —— OUTER JOIN

3 The connection of the above tables

Cross connect —— CROSS JOIN

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coupling （ Join tables in columns ）

● coupling （ JOIN ） Is to add columns from other tables , Conduct “ Add columns ” Set operation of .UNION It's to do （ The longitudinal ） Operate for units , And the connection is by column （ The transverse ） In units .
● Connections are generally divided into internal connections and external connections . First of all, please firmly grasp the use of these two connections .

What is connection

• coupling （ JOIN ） Simply speaking , Is to add columns from other tables , Conduct “ Add columns ” Arithmetic , By using joins, you can select data from multiple tables
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Internal connection —— INNER JOIN

surface 1

surface 2 

As shown in the table above , The columns in the two tables can be divided into the following two categories

 

  The so-called connection operation , In a word , Namely “ With A Columns in serve as bridges , take B The columns that meet the same conditions in are collected into the same result ”

SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price
FROM ShopProduct AS SP INNER JOIN Product AS P 
ON SP.product_id = P.product_id;

You can find that the two tables are combined （ps: You must use ON Clause , And write in FROM and WHERE Between , When using connections SELECT The columns in the clause need to follow “ < The table alias > . < Name > ” Write in the format of .)

Extraneous junction —— OUTER JOIN

Outreach is also through ON The join key of clause joins the two tables , And select the corresponding columns from the two tables at the same time . The basic method of use is no different , But the result is different .

SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price
FROM ShopProduct AS SP RIGHT OUTER JOIN Product AS P 
ON SP.product_id = P.product_id;

  Inline join can only select data that exists in two tables at the same time , For outreach , As long as the data exists in a table , Can read out , The final result will include all the data in the main table . The key to specify the main table is LEFT and RIGHT . seeing the name of a thing one thinks of its function , Use LEFT when FROM The table written on the left in the clause is the main table , Use RIGHT The table on the right is the main table

3 The connection of the above tables

surface 3

SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name, P.sale_price, IP.inventory_quantity
  FROM ShopProduct AS SP INNER JOIN Product AS P
    ON SP.product_id = P.product_id
               INNER JOIN InventoryProduct AS IP
                   ON SP.product_id = IP.product_id
WHERE IP.inventory_id = 'S001';

Cross connect —— CROSS JOIN

Cross join is the basis of all join operations .

SELECT SP.shop_id, SP.shop_name, SP.product_id, P.product_name
FROM ShopProduct AS SP CROSS JOIN Product AS P;

  The set operator to cross join tables that meet the same rules is CROSS JOIN （ Cartesian product ）. You cannot use the... Used in inner and outer joins when making cross joins ON Clause , This is because cross join is the cross combination of all records in two tables , So the number of records in the result is usually the product of the number of rows in two tables .

An inline connection is part of a cross connection ,“ Inside ” It can also be understood as “ The part contained in the cross join result ”. contrary , Extraneous “ Outside ” It can be understood as “ The part beyond the cross connect result ”.