Mysql database learning 2

EXP {MM/ YYYYY} 2020-11-10 12:48:08
mysql database learning


MySQL Database learning 2(SQLyog)

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One 、DQL Language learning ( Two )

Learning content :

Other knowledge points
(1) Query the structure of a known table , Use keywords desc
give an example :

 DESC employees;

Running results :
 Insert picture description here
Basic query ( Make up a section of knowledge )
# Splicing function concat( Late learning function , I'll also talk about )
(1) and c++ Language is similar to , The key words are used in splicing concat
give an example :




SELECT CONCAT ('a','b','c') AS result ;
SELECT CONCAT (last_name,first_name) AS full name FROM employees;

Running results :
 Insert picture description here
 Insert picture description here
Conditions of the query
grammar :



select

 Query list

from

 Table name

where

 filter

classification :
One 、 Filter by conditional expression

Conditional expression operators :
< 、>、 =、 !=( It's not equal to )、 <>(MySQL Is not equal to )、 >=、 <=

# Case a
Query salary greater than 12000 All employee information for

SELECT
*
FROM
employees
WHERE salary > 12000 ;

Running results :
 Insert picture description here # Case 2
Query department number is not equal to 90 The employee's name and department number

SELECT 
last_name,
department_id 
FROM
employees 
WHERE department_id <> 90 ;

Running results :
 Insert picture description here

Two 、 Filter by logical expression

Logical operators :
&&、 ||、 !( The same applies )
and 、or 、not( yes MySQL Standard format in )

# Case a
Check salary at 10000 To 20000 Between the employee's name 、 Salary and bonus

SELECT 
last_name,
salary,
commission_pct 
FROM
employees 
WHERE salary >= 10000 
AND salary <= 20000 ;

Running results :
 Insert picture description here
# Case 2
The inquiry department number is not in 90 To 110 Between , Or pay more than 15000 And employee information .


SELECT 
* 
FROM
employees 
WHERE NOT (
department_id >= 90 
AND department_id <= 110
) 
OR salary > 15000 ;

Running results :
 Insert picture description here

3、 ... and 、 Fuzzy screening

like、between and 、in 、is null 、is not null
#1.like
characteristic :
(1) Can be used with wildcards
% Any number of characters
_ Any single character
# Case a
Query employee name contains characters a Employee information






SELECT
*
FROM
employees
WHERE last_name LIKE '%a%' ;

Be careful : Characters must be in single quotation marks , And contains a There may be characters before and after the description , So you have to have %
Running results :
 Insert picture description here
# Case 2
The third character in the query employee name is e, The fifth character is a The name and salary of the employee



SELECT
last_name,
salary
FROM
employees
WHERE '__e_a%' ;

# Case three
The second character in the query employee name is _ Employee name of

SELECT
last_name
FROM
employees
WHERE last_name LIKE '_\_%' ;

Be careful :_ As a wildcard , have access to \ Translate
Running results :
 Insert picture description here
#2.between and
characteristic :
(1) Contains a critical value
(2)and The values at both ends cannot be exchanged
# Case a
Query the employee number in 100 To 120 Employee information between







SELECT
*
FROM
employees
WHERE employee_id BETWEEN 100
AND 120 ;

Running results :
 Insert picture description here
#3.in
# Case a
The job number of the employee is IT_PROG、AD_VP、AD_PRES An employee's name and job number in the



SELECT
last_name,
job_id
FROM
employees
WHERE job_id IN ('IT_PROG', 'AD_VP', 'AD_PRES') ;

Running results :
 Insert picture description here
#4.is null
# Case a
Query the employee name and bonus rate without bonus



SELECT
last_name,
commission_pct
FROM
employees
WHERE commission_pct IS NULL ;

Running results :
 Insert picture description here
Be careful :Tip:<=> For safety, it is equal to the symbol , You can judge null Value can be used to judge the normal value , Equivalent to an equal sign, but less readable .

# Test questions
#1. There is no bonus for the query , And the salary is less than 18000 Salary and employee name

SELECT
salary,
last_name
FROM
employees
WHERE commission_pct IS NULL
AND salary < 18000 ;

#2. Inquire about employees In the table , The job number is not ’IT’ Or the salary is 12000 Employee information

SELECT
*
FROM
employees
WHERE job_id <> 'IT'
OR salary = 12000 ;

#3. Check out the Department departments The structure of the table

DESC departments;

#4. Inquiry Department departments What position numbers are involved in the table

SELECT DISTINCT
location_id
FROM
departments ;
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