MySQL learning record (6)

1. View

Definition ： View is a virtual table , rely on select Sentence creation

Reason for creating view ：

1. By defining views, frequently used SELECT Statement save for efficiency .
2. By defining views, users can see data more clearly .
3. By defining the view, you can not expose all the fields of the data table , Enhance data confidentiality .
4. Data redundancy can be reduced by defining views .

1.1 Syntax for creating views

CREATE VIEW < View name >(< Name 1>,< Name 2>,...) AS <SELECT sentence >

among SELECT Sentences need to be written in AS After keyword . SELECT The columns in the statement are arranged in the same order as the columns in the view , SELECT Regulation in statement 1 The column is the... In the view 1 Column , SELECT Regulation in statement 2 The column is the... In the view 2 Column , And so on . And the column name of the view is defined in the list after the view name .
View name In the database, it needs to be only Of , Cannot have the same name as other views and tables .

When defining a view Not to be used ORDER BY sentence

Create view ：

Views based on a single table

CREATE VIEW productsum (product_type, cnt_product)
AS
SELECT product_type, COUNT(*)
  FROM product
 GROUP BY product_type ;

Views based on multiple tables

-- First create a new table 
CREATE TABLE shop_product
(shop_id    CHAR(4)       NOT NULL,
 shop_name  VARCHAR(200)  NOT NULL,
 product_id CHAR(4)       NOT NULL,
 quantity   INTEGER       NOT NULL,
 PRIMARY KEY (shop_id, product_id));
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000A',	' Tokyo ',		'0001',	30);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000A',	' Tokyo ',		'0002',	50);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000A',	' Tokyo ',		'0003',	15);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000B',	' Nagoya ',	'0002',	30);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000B',	' Nagoya ',	'0003',	120);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000B',	' Nagoya ',	'0004',	20);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000B',	' Nagoya ',	'0006',	10);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000B',	' Nagoya ',	'0007',	40);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000C',	' Osaka ',		'0003',	20);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000C',	' Osaka ',		'0004',	50);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000C',	' Osaka ',		'0006',	90);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000C',	' Osaka ',		'0007',	70);
INSERT INTO shop_product (shop_id, shop_name, product_id, quantity) VALUES ('000D',	' Fukuoka, ',		'0001',	100);

-- stay productsum shop_product Create a view based on 
CREATE VIEW view_shop_product(product_type, sale_price, shop_name)
AS
SELECT product_type, sale_price, shop_name
  FROM product,
       shop_product
 WHERE product.product_id = shop_product.product_id;

1.2 Modify the syntax of the view structure

ALTER VIEW < View name > AS <SELECT sentence >
-- Modify the view 
ALTER VIEW productSum
    AS
        SELECT product_type, sale_price
          FROM Product
         WHERE regist_date > '2009-09-11';

1.3 Update the view

The view is a virtual table , Therefore, the operation on the view is the operation on the underlying basic table , Therefore, only when the definition of the underlying basic table is met can the modification be successful .

That is, update and modify the view , It is essentially a modification of the basic table .

For a view , If any of the following structures are included, they cannot be updated ：

• Aggregate functions SUM()、MIN()、MAX()、COUNT() etc. .
• DISTINCT keyword .
• GROUP BY Clause .
• HAVING Clause .
• UNION or UNION ALL Operator .
• FROM Clause contains multiple tables .

-- The statement that updates the view , Use UPDATE  SET
UPDATE productsum
   SET sale_price = '5000'
 WHERE product_type = ' Office Supplies ';

1.4 Delete view

DROP VIEW < View name 1> [ , < View name 2> …]
DROP VIEW productsum;

2. Subquery

Subquery refers to a query that is nested within another query statement by one query statement , The subquery result is used as the outer layer of another query Filter conditions , Queries can be Based on one or more tables .

Subqueries are one-time , It will not be saved as a view .

2.1 nested subqueries

-- nested subqueries 
SELECT product_type, cnt_product
FROM (SELECT *
        FROM (SELECT product_type, 
                      COUNT(*) AS cnt_product
                FROM product 
               GROUP BY product_type) AS productsum
       WHERE cnt_product = 4) AS productsum2;
-- The innermost subquery is named productSum, This statement is based on product_type Group and query the number , In the second level query, the number will be 4 Find out your products , Outermost query product_type and cnt_product Two .
-- For simplicity and efficiency , Try to minimize SELECT Iteration of subquery statements 

2.2 Scalar subquery （ Single subquery ）

Singleness is what is required SQL sentence

-- Query the goods whose sales unit price is higher than the average unit price 
SELECT product_id, product_name, sale_price
  FROM product
 WHERE sale_price > (SELECT AVG(sale_price) FROM product);

Only one value can be returned , That is to return the specific... In the table A column of a row

SELECT product_id,
       product_name,
       sale_price,
       (SELECT AVG(sale_price)
          FROM product) AS avg_price
  FROM product;

 

 2.3 Associated subquery

The associated sub query connects the internal and external queries to filter data

SELECT product_type, product_name, sale_price
  FROM product AS p1
 WHERE sale_price > (SELECT AVG(sale_price)
                       FROM product AS p2
                      WHERE p1.product_type = p2.product_type
   GROUP BY product_type);
-- The internal table is P2, The external table is P1, Use WHERE Connect the two tables together 

  The execution process of association query ：

1. First, execute without WHERE The main query of
2. Match according to the main query result product_type, Get subquery results
3. Combine the sub query results with the main query to execute the complete query SQL sentence

Exercises 1

Create a view that meets the following three conditions （ The view name is ViewPractice5_1）. Use product（ goods ） Table as a reference table , Suppose the table contains the initial state of 8 Row data .

• Conditions 1： Sales unit price is greater than or equal to 1000 Yen .
• Conditions 2： The registration date is 2009 year 9 month 20 Japan .
• Conditions 3： Include product name 、 Sales unit price and registration date .

CREATE VIEW ViewPractice5_1
AS 
(SELECT product_name,sale_price,regist_date
FROM product
WHERE sale_price >= 1000 AND regist_date="2009-09-20" );

  View created in exercise 1 ViewPractice5_1 Insert the following data , What's the result ？

SQL Something went wrong , I don't know the result

-- Please write according to the following results  SELECT  sentence , among  sale_price_all  List the average selling unit price of all goods .

/*product_id | product_name | product_type | sale_price | sale_price_all
------------+-------------+--------------+------------+---------------------
0001       | T shirt          |  clothes          | 1000       | 2097.5000000000000000
0002       |  Punch         |  Office Supplies       | 500        | 2097.5000000000000000
0003       |  motion T T-shirt        |  clothes           | 4000      | 2097.5000000000000000
0004       |  kitchen knife           |  Kitchenware       | 3000       | 2097.5000000000000000
0005       |  pressure cooker         |  Kitchenware       | 6800       | 2097.5000000000000000
0006       |  Fork           |  Kitchenware       | 500        | 2097.5000000000000000
0007       |  Clean the board         |  Kitchenware        | 880       | 2097.5000000000000000
0008       |  Ball pen         |  Office Supplies        | 100       | 2097.5000000000000000*/

SELECT product_id,product_name,product_type,sale_price,
(SELECT AVG(sale_price)
 FROM product) AS sale_price_all
FROM product
ORDER BY product_id;

Please write one according to the conditions in exercise 1 SQL sentence , Create a view with the following data （ The name is AvgPriceByType）.

product_id | product_name | product_type | sale_price | avg_sale_price
------------+-------------+--------------+------------+---------------------
0001       | T shirt          |  clothes          | 1000       |2500.0000000000000000
0002       |  Punch          |  Office Supplies      | 500        | 300.0000000000000000
0003       |  motion T T-shirt         |  clothes         | 4000        |2500.0000000000000000
0004       |  kitchen knife           |  Kitchenware       | 3000        |2795.0000000000000000
0005       |  pressure cooker          |  Kitchenware      | 6800        |2795.0000000000000000
0006       |  Fork           |  Kitchenware       | 500         |2795.0000000000000000
0007       |  Clean the board          |  Kitchenware      | 880         |2795.0000000000000000
0008       |  Ball pen          |  Office Supplies      | 100         | 300.0000000000000000

/* Tips ： The key is  avg_sale_price  Column . Different from exercise 3 , Here we need to calculate   It is the average selling unit price of each commodity type . This is the same as using the associated subquery .  in other words , This column can be created using the associated subquery . The question is where to use this associated subquery .*/
CREATE VIEW AvgPriceByType
AS (SELECT product_id,product_name,product_type,sale_price,
(SELECT avg(sale_price)
from product AS p2
WHERE p1.product_type=p2.product_type
GROUP BY p1.product_type) AS avg_sale_price
FROM product AS p1);