Use of aggregate functions

Catalog

One 、 Aggregate function introduction

1. Concept

2. Type of aggregate function

①AVG and SUM function

②MIN and MAX function

③COUNT function

Ⅰ. How to realize the number of records in the calculation table ？

Ⅱ.AVG = SUM/COUNT

Ⅲ. Query the average bonus rate in the company

Use count(*),count(1),count( Specific fields ) Which is more efficient ?

Two 、GROUP BY Use         

demand 1： Query the average salary of each department , Maximum wage

demand 2： Check each one department_id,job_id The average wage of

​ Conclusion

3、 ... and 、HAVING Use

1. effect

2. practice ： Query the highest salary ratio in each department 10000 High department information

requirement

Four 、SQL The underlying execution principle

（ One ）WHERE And HAVING Comparison of

1. Scope of application

2. If the filter condition has no aggregate function

summary  

（ Two ）SELECT sentence

1.SQL92 The grammar of

2.SQL99 The grammar of

（ 3、 ... and ）SQL Statement execution

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One 、 Aggregate function introduction

1. Concept

Aggregate functions act on a set of data , And return a value for a set of data .

2. Type of aggregate function

①AVG and SUM function

Only applicable to fields of numeric type （ Or variable ）

SELECT AVG(salary),SUM(salary),AVG(salary)*107
FROM employees;
# The following operations are meaningless 
SELECT SUM(last_name),AVG(last_name),SUM(hire_date)
FROM employees;

②MIN and MAX function

For numeric types 、 String type 、 Fields of date time type （ Or variable ）

SELECT MAX(salary),MIN(salary),MAX(hire_date),MIN(hire_date)
FROM employees;

③COUNT function

Calculate the number of specified fields in the query structure

SELECT COUNT(employee_id),COUNT(salary),COUNT(2*salary),COUNT(1)
FROM employees;

Ⅰ. How to realize the number of records in the calculation table ？

The way 1：COUNT(*)

The way 2：COUNT(1)

The way 3：COUNT( Specific fields )  ——  Not necessarily right

Be careful ： When calculating the number of occurrences of the specified field , It is not calculated NULL It's worth it

Ⅱ.AVG = SUM/COUNT

SELECT AVG(salary),SUM(salary)/COUNT(salary),
AVG(commission_pct),SUM(commission_pct)/COUNT(commission_pct),
SUM(commission_pct) / 107
FROM employees;

Ⅲ. Query the average bonus rate in the company

# Wrong writing 
SELECT AVG(commission_pct)
FROM employees;

# Write it correctly 
SELECT SUM(commission_pct) / COUNT(IFNULL(commission_pct,0)),
AVG(IFNULL(commission_pct,0))
FROM employees;

Use count(*),count(1),count( Specific fields ) Which is more efficient ?

If you are using MYISAM Storage engine , Then the efficiency of the three is the same , All are O（1）

If you are using InnoDB Storage engine , Then the efficiency of the three count(*) = count(1) > count( Specific fields )

Two 、GROUP BY Use         

demand 1： Query the average salary of each department , Maximum wage

SELECT AVG(salary)
FROM employees;   # The average salary of the whole company 

SELECT department_id,AVG(salary),MAX(salary)
FROM employees
GROUP BY department_id;

demand 2： Check each one department_id,job_id The average wage of

SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id,job_id;

Conclusion

1.SELECT Fields of non group functions appearing in , It must be stated that in GROUP BY in . conversely ,GROUP BY Fields declared in can not appear in SELECT in  

2.GROUP BY The statement in FROM Back 、WHERE Back ,ORDER BY front 、LIMIT front  

3. stay MySQL Use in WITH ROLLUP： Add a record after all the queried grouping records , This record Calculate the sum of all the records queried , That is, count the number of records .

3、 ... and 、HAVING Use

1. effect

Filtering data

2. practice ： Query the highest salary ratio in each department 10000 High department information

# Wrong writing 
SELECT department_id,MAX(salary)
FROM employees
WHERE MAX(salary) > 10000
GROUP BY department_id;

# Write it correctly 
SELECT department_id,MAX(salary)
FROM employees
GROUP BY department_id
HAVING MAX(salary) > 10000;

requirement

1. If an aggregate function is used in the filter condition , Must be used HAVING To replace WHERE, Otherwise, an error will be reported
2. HAVING It must be stated that in GROUP BY Behind
3. In development , Use HAVING The premise is SQL Used in GROUP BY

Four 、SQL The underlying execution principle

When there is an aggregate function in the filter condition , Then this filter condition must be declared in HAVING in

When there is no aggregate function in the filter condition , Then this filter condition is declared in WHERE Medium or HAVING Either way , however , It is suggested that you declare in WHERE in                                

（ One ）WHERE And HAVING Comparison of

1. Scope of application

HAVING Has a wider scope of application

2. If the filter condition has no aggregate function

WHERE More efficient than HAVING

summary  

（ Two ）SELECT sentence

1.SQL92 The grammar of

SELECT ......( There are aggregate functions )

FROM ......

WHERE  Connection conditions of multiple tables   AND  Filter conditions that do not contain aggregate functions

GROUP BY ......

HAVING  Filter conditions containing aggregate functions

ORDER BY ......(ASC / DESC)

LIMIT ......

2.SQL99 The grammar of

SELECT ......( There are aggregate functions )

FROM ....  (LEFT / RIGHT) JOIN ... ON Connection conditions of multiple tables

(LEFT / RIGHT) JOIN ... ON ...

WHERE  Filter conditions that do not contain aggregate functions

GROUP BY ......

HAVING Filter conditions containing aggregate functions

ORDER BY ......(ASC / DESC)

LIMIT ......

（ 3、 ... and ）SQL Statement execution

FROM ......  -->  ON  -->  (LEFT / RIGHT  JOIN)  -->  WHERE  -->  GROUP BY  -->  HAVING  --> SELECT  -->  DISTINCT（ duplicate removal ）  -->  ORDER BY（ Sort ）  -->  LIMIT（ Pagination display ）