MySQL learning record (5)

1. Aggregate functions

1.1SQL The function used for aggregation in is called aggregate function . The following five are the most commonly used aggregate functions ：

• COUNT： Count the number of records in the table （ Row number ）
• SUM： Calculate the total value of the data in the numerical column of the table
• AVG： Calculate the average value of the data in the numerical column in the table
• MAX： Find the maximum value of the data in any column of the table
• MIN： Find the minimum value of data in any column in the table

practice ：

--  Calculate the number of rows of all data （ contain NULL） 
SELECT COUNT(*) FROM product; 
--  Calculation NULL Number of rows of data other than  
SELECT COUNT(purchase_price) FROM product; 
--  Calculate the total value of sales unit price and purchase unit price  
SELECT SUM(sale_price), SUM(purchase_price) FROM product; 
--  Calculate the average of sales unit price and purchase unit price  
SELECT AVG(sale_price), AVG(purchase_price) FROM product; 
-- MAX and MIN It can also be used for non numerical data  
SELECT MAX(regist_date), MIN(regist_date) FROM product; 

1.2 Use aggregate functions to eliminate duplicate values

--  Calculate the number of data rows after removing duplicate data 
SELECT COUNT(DISTINCT product_type)
 FROM product;
 
--  Whether to use DISTINCT The difference in action （SUM function ）
SELECT SUM(sale_price), SUM(DISTINCT sale_price)
 FROM product;

1.3 matters needing attention

• COUNT The result of the function varies according to the parameters .COUNT(*) It will contain NULL The number of rows of data , and COUNT(< Name >) You'll get NULL Number of rows of data outside .
• The aggregate function will NULL Ruled out . but COUNT(*) exception , Does not rule out NULL.
• MAX/MIN The function applies to columns of almost all data types .SUM/AVG The function is only applicable to columns of numeric type .
• When you want to calculate the type of value , Can be in COUNT In the parameters of the function DISTINCT.
• Use... In the arguments of aggregate functions DISTINCT, Duplicate data can be removed .

2. Group tables

2.1GROUP BY sentence （ grouping ）

--group by grammar 
SELECT < Name 1>,< Name 2>, < Name 3>, ……
  FROM < Table name >
 GROUP BY < Name 1>, < Name 2>, < Name 3>, ……;

--  Count the number of rows of data by commodity category 
SELECT product_type, COUNT(*)
  FROM product
 GROUP BY product_type;
--GROUP BY The columns in the clause are called aggregate columns or grouping keys 

 --  Not included GROUP BY
SELECT product_type, COUNT(*)
  FROM product

 2.2GROUP BY Statement writing position

GROUP BY There are strict requirements for the writing order of clauses , Failure to comply with the requirements will result in SQL Unable to execute normally , The clauses that have appeared so far Writing **** The order by ：

1**.**SELECT → 2. FROM → 3. WHERE → 4. GROUP BY

The first three items are used to filter the data ,GROUP BY Process the filtered data

SELECT purchase_price, COUNT(*)
  FROM product
 WHERE product_type = ' clothes '
 GROUP BY purchase_price;

 2.3 Common mistakes

When using aggregate functions and GROUP BY When clause , Common mistakes are ：

1. At the end of the aggregate function SELECT Clause writes columns other than aggregate keys Use COUNT Wait for the aggregate function ,SELECT If the column name appears in the clause , Can only be GROUP BY The column name specified in clause （ That's the aggregate bond ）.
2. stay GROUP BY Use column aliases in clause SELECT Clause can be passed through AS To specify an alias , But in GROUP BY Alias cannot be used in . Because in DBMS in ,SELECT Clause in GROUP BY Execute after clause .
3. stay WHERE Aggregate functions are used in The reason is that the aggregate function is used only if the result set has been determined , and WHERE Still in the process of determining the result set , So contradictions can lead to errors . If you want to specify conditions , Can be in SELECT,HAVING（ I'll talk about ） as well as ORDER BY The aggregate function is used in clause .

3. Specify conditions for aggregate results

3.1 utilize HAVING Get specific groups

HAVING Usage is similar. WHERE , But the two are different for the object ,HAVING For group ,WHERE For the line （ Record ）. 

3.2HAVING characteristic

HAVING Clause is used to filter groups , have access to Numbers 、 Aggregate functions and GROUP BY The column name specified in （ Polymerization bond ）. 

--  Numbers 
SELECT product_type, COUNT(*)
  FROM product
 GROUP BY product_type
HAVING COUNT(*) = 2;
--  Wrong form （ because product_name Not included in GROUP BY In the aggregate bond ）
SELECT product_type, COUNT(*)
  FROM product
 GROUP BY product_type
HAVING product_name = ' Ball pen ';

4. Sort query results

4.1  ORDER BY

SQL The execution results in are arranged randomly , When you need to sort in a specific order , Can be used ORDER BY Clause .

ORDER BY grammar ：

SELECT < Name 1>, < Name 2>, < Name 3>, ……
  FROM < Table name >
 ORDER BY < Sort base column 1>, < Sort base column 2>, ……
-- Default ascending order   namely ASC array 
-- The descending order is DESC
--  Descending order 
SELECT product_id, product_name, sale_price, purchase_price
  FROM product
 ORDER BY sale_price DESC;
--  Multiple sort keys 
SELECT product_id, product_name, sale_price, purchase_price
  FROM product
 ORDER BY sale_price, product_id;
--  When the column name used for sorting contains NULL when ,NULL It will be summarized at the beginning or end .
SELECT product_id, product_name, sale_price, purchase_price
  FROM product
 ORDER BY purchase_price;

4.2 ORDER BY Aliases can be used in

SQL In the use of HAVING When clause SELECT Of the statement perform **** The order by ：

FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY

among SELECT The order of execution is GROUP BY After Clause ,ORDER BY Before clause . in other words , When in ORDER BY When using aliases in , I already know SELECT The alias set does not exist , But in GROUP BY When you use aliases in, you don't know the existence of aliases , therefore stay ORDER BY You can use aliases in , But in GROUP BY Alias cannot be used in ****.

/* Exercises 5
 Please point out the following SELECT All the grammatical errors in the statement .*/
SELECT product_id, SUM（product_name）
-- Ben SELECT There is an error in the statement .
  FROM product 
 GROUP BY product_type 
 WHERE regist_date > '2009-09-01';

-- error 1： Wrong aggregate key , GROUP BY The order of execution is SELECT Before ,product_type There is no in select In the sentence 
-- error 2： The writing order is wrong ,WHERE The clause should be in GROUP BY Before 

Exercises 6

Please write one SELECT sentence , Find out the sales unit price （sale_price Column ） The total value is greater than the purchase unit price （purchase_price Column ） Total value 1.5 Times the commodity category . The results are shown below .

 select product_type, sum(sale_price), sum(purchase_price)
 from product
WHERE (select sum(sale_price)>sum(purchase_price)*1.5);
-- Different from the results given 

  It should be according to the date regist_date In descending order

ORDER BY regist_date DESC;