Chapter 8 aggregate function

Chapter viii. Aggregate functions
Aggregate function type ：AVG(),SUM(),MAX(),MIN(),COUNT()

AVG(),SUM() Only suitable for variables and fields of numeric type , String cannot be summed or averaged

SELECT AVG(salary),MAX(salary),MIN(salary),SUM(salary)
FROM employees

MIN(),MAX() For numeric types , String type , String or variable of date time type

SELECT MAX(last_name),MIN(last_name),MAX(hire_date),MIN(hire_date)
FROM employees

COUNT: The function is to calculate the number of specified fields in the query structure , It is not calculated NULL It's worth it

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

# How to calculate the number of records in a table
# Mode one ：count(*) Mode two ：count(1)

# Check the name and bonus rate of employees with bonuses
SELECT last_name, commission_pct
FROM employees
WHERE commission_pct IS NOT NULL

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

# because count Not in the count Null value , So want to use ifnull
SELECT SUM(commission_pct)/COUNT(IFNULL(commission_pct,0)),
AVG(IFNULL(commission_pct,0))
FROM employees;

How to count records , Sure COUNT(),COUNT(*),COUNT( Use no non empty fields )
Which is more efficient
use MYISAM The efficiency of the three engines is equal
use INNODB The engine of COUNT(*)=COUNT(1) Better than COUNT( Field )

#2 group by Use : Group by field

# Check the average and maximum wages of each department
SELECT department_id,AVG(salary),SUM(salary)
FROM employees
GROUP BY department_id
ORDER BY department_id ASC;

# demand : Check each one job_id The average wage of
SELECT job_id,AVG(salary),SUM(salary)
FROM employees
GROUP BY job_id;

# demand ： Ask different departments , identical job The average wage of

# Ask different departments first , Find a different job
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id,job_id;

# perhaps ： Make a difference first job_id, Ask for different departments Because the above two divisions are the same type of work and the same department number, they will be together , It's just the difference between first and then
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id,job_id;

# The following is wrong
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id;

# Conclusion :select The fields of non group functions appearing in must be declared in group by in
# conversely ,group by Declared fields may not appear in select in

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

# Query the average salary of each department , In ascending order of average wage
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC;

#having effect ： It is used to filter data , I have learned to filter data before where,having And group by of

# Query the highest salary ratio in each department 10000 High department information
# Demand one : Once a group function appears in the filter condition group by, Aggregate functions , Then the filter condition must be having Replace , Out of commission where;
# Requirement 2 ：having It must be stated that in group by Behind ,having Must be with group by Use it together
# The following is wrong : If there are two conditions ,where And having Can be used at the same time
SELECT department_id,MAX(salary)
FROM employees
WHERE MAX(salary)>10000
GROUP BY department_id;

# The following is correct
SELECT department_id,MAX(salary)
FROM employees
GROUP BY department_id
HAVING MAX(salary)>10000;

# Inquiry Department id by 10,20,30,40 The highest wage ratio of these four departments 10000 High department information
# Mode one
SELECT department_id,MAX(salary)
FROM employees
WHERE department_id IN(10,20,30,40)
GROUP BY department_id
HAVING MAX(salary)>10000;
# Mode two
SELECT department_id,MAX(salary)
FROM employees
GROUP BY department_id
HAVING MAX(salary)>10000 AND department_id IN(10,20,30,40);