Mysql database foundation: DQL data query language

This chapter is pure notes

DQL Language

brief introduction

DQL（data query language） Data query language , Specifically used to query data .

1、 Basic query

1.1 grammar

Query the specified fields in the specified table 、 Constant etc.

SELECT  Query list  FROM  Table name ;

Queries all fields in the specified table 、 Constant etc.

SELECT * FROM  Table name ;

Display table structure

DESC  Table name ;

1.2 Basic query features

1、 The query list can be fields in the table 、 Constant 、 expression 、 function

2、 The result of the query is a virtual table

3、 There can be multiple fields in the query list , Separated by commas

1.3 mark of emphasis ``（ Double inverted quotation marks ） Use

Can be used to Distinguish between fields and keywords , Such as ：

SELECT `NAME` FROM  A table ;

1.4 Query constant value

SELECT 100;
SELECT 'John';

1.5 Query expression

SELECT 100%98;

1.6 Query function

SELECT VERSION(); #VERSION() Is the function , The function is to show mysql edition 

1.7 Alias fields

# Mode one   Use AS
SELECT 100%98 AS  result ;  #  result ：2

SELECT last_name AS  surname , first_name AS  name  FROM  Table name ; 

# Mode two   You can omit AS
SELECT last_name  surname ,first_name  name  FROM  Table name ;

The benefits and fallibility of aliasing

1. Easy to understand
2. If the query field has the same name , Use aliases to distinguish
3. It's better not to take a space in the alias , If there is a space , Can be modified with double quotation marks
   for example ：

# Suppose you want to query salary salary, The result is out put, With space in the middle , Use double quotation marks out put  Lead up 
SELECT salary AS "out put" FROM  Table name ;

1.8 duplicate removal —distinct

# Remove the redundant data that appears repeatedly in the field 
SELECT DISTINCT  Field  FROM  Table name ;

1.9 “+” Use of numbers

effect ： There's only one function , Add as an operator

# Both operands are numeric , Then add directly 
SELECT 100+100; 

# One of them is character type , Trying to convert a character value to a numeric value , If the conversion is successful , Then add , If you fail , Then convert the character value to 0
SELECT '123' + 90;

# As long as one of them is null, The result must be null
SELECT null + 10;   # The result is null

2、 Conditions of the query

2.1 grammar

where Keywords are used to give query conditions .

SELECT 
	 Query list ;
FROM 
	 Table name ;
WHERE
	 filter ;

2.2 classification

2.2.1 Filter by conditional expression

Conditional operator ：>（ Greater than ）< （ Less than ） =（ be equal to ） !=( It's not equal to ) <>( It's not equal to )

# Case study 1： Query salary >12000 Employee information 
SELECT
		*
FROM
		employees
WHERE
		salary>12000;  # Conditions 
		
# Case study 2： Query department number is not equal to 90 The employee name and department number of the 
SELECT 
		last_name,
		department_id
FROM	
		employees
WHERE 	
		department_id <> 90;    #<>  Or you could write it as !=

2.2.2 Filter by logical expression

Logical operators ：

Logical operators	effect
&&	And
||	or
!	Not
and	Word form “ And ”
or	Word form “ or ”
not	Word form ” Not “

effect ： Join conditional expressions

# Case study 1： 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;
	
# Case study 2： The inquiry department number is not in 90 To 110 Between , Or pay more than 15000 Employee information 
SELECT	
		*
FROM
		employees
WHERE
		NOT(department_id >= 90 AND department_id <= 110) OR salary>15000;

#  Or as follows 
select
		*
FROM
		employees
WHERE
		department_id NOT BETWEEN 90 AND 110
OR
		salary>15000;

2.2.3 Fuzzy query

1、like

characteristic ： Generally used with wildcards .

wildcard	effect
%	Represents any number of characters
_	Underline , Represents any single character
/	Escape character

# Case study 1： Query employee hit contains characters a Employee information 
SELECT
		*
FROM
		employees
WHERE
		last_name LIKE '%a%';

# Case study 2： The third character in the query employee name is e, The fifth character is a Employee name of 
SELECT
		last_name
FROM
		employees
WHERE
	last_name LIKE '__e_a%';

# The second character in the employee name is _ Employee name of 
SELECT
		last_name
FROM
		employees
WHERE
		last_name LIKE '_\_%';
# Or it could be written as ：last_name LIKE '_$_%' ESCAPE '$';

2、between…and…

meaning ： The expression is in the middle of the two expressions , If you add not It means not in the middle .

characteristic ：

• Use between and It can improve the conciseness of sentences
• Contains a critical value

SELECT
		*
FROM	
		employees
WHERE
		employee_id >= 100 AND employee_id <= 120;
# The above code is as useful as the following code 

SELECT	
		*
FROM
		employees
WHERE
		emoloyees_id between 100 AND 120;

3、in

meaning ： Determine whether the value of a field belongs to in An item in the list .
characteristic ：

• It can improve the introduction of sentences
• in The value types of the list must be uniform or compatible
• I won't support it in The values in the list plus wildcards

# The job number of the employee is IT_PROT,AD_VP An employee's name and job number in the 
SELECT
		last_name,job_id
FROM
		employees
WHERE
		job_id IN('IT_PROT','AD_VP');

4、is null and is not null

Be careful ：
= or != or <> Can not be used to judge null value , and adopt is null and is not null Can be judged null value

# Query the employee name and bonus rate without bonus 
SELECT
		last_name,commission_pct
FROM
		employees
WHERE	
		commission_pct IS NULL;

5、 Safety is equal to <=>

SELECT
		last_name,salary
FROM	
		employees
WHERE	
		salary <=> 12000;

is null and <=> difference ：

IS NULL： Can be judged NULL value , High readability
<=>： You can judge NULL value , You can also judge the normal value , Poor readability .

3、 Sort query

3.1 grammar

SELECT
		 Query list 
FROM	
		 Table name 
WHERE	
		 filter 
ORDER BY
		 Sort list (asc \ desc)
		#asc Ascending 、desc Descending , Default ascending order 

3.2 Examples demonstrate

# Case study 1： Query department number is greater than or equal to 90 Employee information , Sort according to the employment time 【 By filter criteria 】
SELECT
		 *
FROM
		employees
WHERE
		department_id>=90
ORDER BY
		hiredate  ASC;
		
# Case study 2： Display employee information and salary by annual salary   Annual salary 【 Query by expression 】
SELECT
		*,salary*12*(1 + IFNULL(commission_pct, 0))  Annual salary 
FROM
		employees
ORDER BY
		salary*12*(1 + IFNULL(commission_pct, 0)) DESC;
		
# Case study 3： Display employee information and salary by annual salary   Annual salary 【 Query by alias 】
SELECT
		*,salary*12*(1 + IFNULL(commission_pct, 0))  Annual salary 
FROM
		employees
ORDER BY
		 Annual salary  DESC;
		
# Case study 4： Display the employee's name and salary by the length of the name 【 Sort by function 】
SELECT 
		LENGTH(last_name)  bytes , last_name, salary
FROM 
		employees
ORDER BY
		LENGTH(last_name);

# Case study 5： Search for employee information , It is required that the salary be increased first , Then sort by employee number in descending order 【 Sort by multiple fields 】
SELECT 
		*
FROM 
		employees
ORDER BY 	
		salary ASC, employees_id DESC;

4、 Common function

4.1 function

Be similar to Java The method in , Encapsulating a set of logical statements in a method body , Name of external exposure method .

4.2 benefits

1、 Hide implementation details ,
2、 Improve code reusability

4.3 Call syntax

SELECT  Function name ( Argument list ) 【FROM  surface 】;

4.4 classification ：

1、 One line function

Such as ：concat、length、isfull etc.

2、 Group function

For statistical use , It's also called statistical function , Aggregate functions , Group functions .

4.5 Introduction to single line functions

4.5.1 Character functions

#length  Gets the number of bytes of the parameter value 
#----------------------------------------------
SELECT LENGTH('john');  #4
SELECT LENGTH(' Zhang Sanfeng hahaha');  #15

#concat  Concatenate characters 
#----------------------------------------------
SELECT CONCAT(last_name,'_',first_name) FROM employees;

#upper\lower  Convert letters to case 
#----------------------------------------------
SELECT UPPER('john');
SELECT LOWER('joHN');

#substr\substring
#-----------------------------------------------
# Pay attention to the index from 1 Start , That is, the first index number is 1
# Intercept all characters after the specified index 
SELECT SUBSTR(' Li Mochou fell in love with Lu Zhanyuan ',7) AS out_put;  # Output ： Lu Zhanyuan 
# Intercepts the characters of the specified length at the specified index 
SELECT SUBSTR(' Li Mochou fell in love with Lu Zhanyuan ',1,3) AS out_put;   # Output ： Li Mochou 

#instr  Returns the index of the first occurrence of the substring , Return if not found 0
#-----------------------------------------------
SELECT INSTR(' Don't try so hard , Use your brain ',' mind ') AS out_put; #  result ：12

#trim  Remove the specified characters on both sides 
#-----------------------------------------------
SELECT LENGTH(TRIM('  Zhang Cuishan '    )) AS out_put;  # result ： length 9, The blanks on both sides are removed , The Chinese character bytes are 3byte, So the length is 9
SELECT TRIM('a' FROM 'aaaaaa Zhang aaaaaaa Cuishan aaaaa') AS out_put; # result ： Zhang aaaaaaa Cuishan  （ Removed both sides a）

#lpad  Fill the specified length with the specified characters 
#-----------------------------------------------
SELECT LPAD(' YanSuSu ',10,'*') AS out_put; #  result ：******* YanSuSu 

#rpad  Fill the specified length with the specified characters 
#-----------------------------------------------
SELECT RPAD(' YanSuSu ',10,'*') AS out_put; #  result ： YanSuSu *******

#replace  Replace 
#-----------------------------------------------
SELECT REPLACE(' Zhouzhiruo zhouzhiruo ',' Zhou Zhiruo ',' Zhao Min ') AS out_put; /* Zhaomin replaces zhouzhiruo */
#  result ： Zhaomin Zhaomin 

4.5.2 Mathematical functions

#round  rounding 
#------------------------------------------------
SELECT ROUND(1.45); /* rounding */ #  result ：1

SELECT ROUND(1.567,2); /* Take two decimal places */ #  result ：1.57

#ceil  Rounding up , Returns the smallest integer greater than or equal to the parameter 
#------------------------------------------------
SELECT CEIL(-1.02); #  result ：-1

#floor  Rounding down , Returns the maximum integer less than or equal to the parameter 
#------------------------------------------------
SELECT FLOOR(-9.99); #  result ：-10

#truncate  Truncate from decimal places , Do not round off 
#------------------------------------------------
SELECT TRUNCATE(1.69999,1); #  result ：1.6

#mod  Remainder 
#------------------------------------------------
SELECT MOD(10,3); # 1
#  Add ： The method of finding the remainder ： hypothesis MOD(a,b) = a-a/b*b;

4.5.3 Date function

# now  Returns the current system date + Time 
SELECT NOW();
#  Or this 
SELECT SYSDATE();

# curdate  Returns the current system date , Not including time 
SELECT CURDATE();

# curtime  Return to current system time , Excluding date 
SELECT CURTIME();

#  Gets the specified part , year , month , Japan , Hours , minute , second 
SELECT YEAR(NOW())  year ; #  result ：2022
SELECT YEAR('2022-1-1')  year ; #  result ：2022

SELECT MONTH(NOW())  month ; # 4
SELECT MONTHNAME(NOW())  month ; # April

# last_day  Get the last day of the current month 
SELECT LAST_DAY(SYSDATE());

# str_to_date: Converts a character to a date in a specified format （ Generally, the returned characters are of character type , So this function is used to convert characters to standard dates 
SELECT STR_TO_DATE('1994-3-3','%Y-%c-%d') AS out_put; # Output ：1994-03-3

#  Example 
SELECT * FROM employees where hiredate = STR_TO_DATE('4-3 1994','%c-%d %Y');
# Will be 4-3 1994 convert to 1994-4-3 This form 

# date_format： Convert date to character 
SELECT DATE_FORMAT(NOW(),'%Y year %m month %d Japan ') AS out_put;

#  Example   Check the name and date of the employee with bonus （xx month xx Japan  %y year ）
SELECT last_name, DATE_FORMAT(hiredate, '%m month %d Japan  %y year ')  Date of entry ;
FROM employees
WHERE commission_pct IS NOT NULL;

4.5.4 Other functions

# Query the current version number 
SELECT VERSION();

# Query the user 
SELECT USER();

4.5.5 Process control functions

# if function ：
#  If the comparison result is true, select the left , Select the right side for the false 
SELECT IF(10<5,' Big ',' Small '); #  Small 
#  for example ：
SELECT last_name,commission_pct,IF(commission_pct IS NULL,' No bonus , ha-ha ',' Bonus , Joy ')  remarks  FROM employees; 

# case  function 
# Use 1： case Be similar to switch The effect of 
/* case  The field or expression to determine  when  Constant 1 then  The value to display 1 Or words 1( There is no semicolon at the end ) when  Constant 2 then  The value to display 2 Or words 2 ... else  The value to display n Or words n end AS  name  FROM  Table name ; */
#  for example ：
/*  Case study ：  Check the salary of employees , The requirements are as follows ：  Department number =30, The wage shown is 1.1 times   Department number =30, The wage shown is 1.2 times   Department number =40, The wage shown is 1.3 times   Other departments , The wage shown is the original wage  */
#  Realization ：
SELECT 
		salary  The original wage ,department_id,
CASE 
		department_id
WHEN 30 THEN salary*1.1
WHEN 40 THEN salary*1.2
WHEN 50 THEN salary*1.3
ELSE 
		salary
END AS  New pay 
FROM 
		employees;
#  The new salary is listed at the end of the queried table 

# Use 2： Similar to multiple if
/* case when  Conditions 1 then  The value to display 1 Or words 1 when  Conditions 2 then  The value to display 2 Or words 2 ... else  The value to display n Or words n end */

#  for example ：
/* Case study ：  Check the salary of employees   If wages >20000, Show A Level   If wages >15000, Show B Level   If wages >10000, Show C Level   Otherwise, it will show D Level  */
#  Realization 
SELECT 
		salary  Wages ,
CASE
WHEN salary>20000 THEN 'A'
WHEN salary>15000 THEN 'B'
WHEN salary>10000 THEN 'C'
ELSE 
		'D'
END AS  Salary level 
FROM 
		employees;

4.6 Group function

sum Sum up ,avg Average ,max Maximum ,min minimum value ,count Calculate the number of

#1、 Easy to use 
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT COUNT(salary) FROM employees;

#2、 Which types of parameters are supported 
#sum and avg Suitable for numeric types 
#max,min,count Suitable for any type 

#3、 Grouping functions ignore null value 

#4、 and distinct Collocation de duplication 
SELECT SUM(DISTINCT salary) FROM employees;

#5、count A separate introduction to functions 
# In general use count(*) Used to count lines , perhaps count(1)
SELECT COUNT(*) FROM employees;

#6、 There are restrictions on the fields that can be queried with the grouping function , The requirement is group by Fields after 

5、 Group query

5.1 grammar

select  Group function , Column （ The requirement appears at the same time group by Behind ）

from  Table name 

group by  A list of groups 

order by  Clause ;

5.2 Easy to use

# Check the maximum salary for all jobs 
SELECT 
	MAX(salary),job_id
FROM 
	employees
GROUP BY 
	job_id;

5.3 Add filter

# Case study 1
# There are... In the query message a Letter , The average salary of all departments 
SELECT 
	AVG(salary), department_id
FROM 
	employees
WHERE 
	email LIKE "%a%"  # Add filter 
GROUP BY 
	department_id;

# Case study 2
# Query the number of employees in each department >2
SELECT 
	COUNT(*), department_id
FROM 
	employees
GROUP BY 
	department_id
HAVING 
	count(*)>2; # Add filter 

5.4 characteristic ：

1、 The screening criteria are divided into two categories

Keywords used	Screening timing	data source	Location
where	Filter before grouping	Original table	group by Before clause
having	Filter after grouping	Grouped result set	group by After Clause

2、 The condition of grouping function must be having clause , If you can use pre group filtering, you can use pre group filtering

3、where and having It is also used to filter conditions , What's the difference?
（1）where and having They all do conditional screening
（2）where The execution time is longer than having Early
（3）where Group functions cannot appear after
（4）having Group functions can appear later
（5）where The statement should be followed by from Back
（6）having The statement should be followed by group by Back

4、 Be careful ：
（1）where The back must be 【 You can't 】 Group functions appear
（2） If select、having A group function appears after the statement , that select、having Columns that are not modified by group functions , Must appear in group by Back （ It's very important ）

6、 Link query

6.1 meaning

It is also called multi table query , When the fields of a query come from multiple tables , You'll use join queries .

Cartesian product ： surface 1 Yes m That's ok , surface 2 Yes n That's ok , result =m*n That's ok

Cause of occurrence ： There are no valid connection conditions

How to avoid ： Add valid connection conditions

6.2 classification

By age

• sql192 standard ： Only internal connections are supported
• sql199 standard （ recommend ）： Support internal connection , External connection （ Right outer connection and left outer connection ,mysql All external connections are not supported in , Cross connect ）

By function

• Internal connection

1. Equivalent connection
2. Non equivalent connection

• External connection

1. Right connection
2. The left outer join
3. Full outer join

• Cross connect

6.3 sql192 standard

6.3.1 Equivalent connection

# Case study 1
# Query the employee name and the corresponding department name 
SELECT 
	last_name,department_name
FROM 
	employees,departments
WHERE 
	employees.`department_id`=departments.`department_id`;
# Note the back quotes 

# Case study 2
# Name the table 
/* 1、 Improve sentence conciseness  2、 Distinguish multiple fields with duplicate names   Be careful ： If you alias a table , The query field cannot use the original table name  */
# Query employee name , official account , Name of work 

SELECT 
	e.last_name,e.job_id,j.job_title
FROM 
	employees e, jobs j
WHERE 
	e.`job_id`=j.`job_id`;

# Case study 3
# Plus screening 
# Query the name of the employee with bonus , Department name 
SELECT 
	last_name, department_name,commission_pct
FROM 
	employees e, departments d
WHERE 
	e.`department_id`=d.`department_id`
AND 
	e.`commission_pct` IS NOT NULL;

# Case study 3
# Add groups 
SELECT 
	count(*)  Number ,city
FROM 
	departments d,locations l
WHERE 
	d.`location_id`=l.`location_id`
GROUP BY
	city;

6.3.2 Non equivalent connection

Is to connect the “=” It was replaced by something else such as >,<,between …and… etc.

6.4 sql199 standard

6.4.1 grammar

select  Query list 
from  surface 1  Alias  【 Connection type 】
join  surface 2  Alias 
on  Connection condition 
【where  filter 】
【group by  Grouping conditions 】
【having  filter 】
【order by  Sort list 】

6.4.2 classification

6.4.2.1 Internal connection ： inner

1、 grammar （inner It can be omitted ）

select  Query list 
from  surface 1  Alias 
inner join  surface 2  Alias 
on  Connection condition 

2、 classification
（1） Equivalent connection

# Case study 1： Query employee name , Department name 
SELECT last_name, department_name
FROM employees e
INNER JOIN departments d
on e.`department_id` = d.`department_id`;

# Case study 2： The query name contains e Name of the employee and type of work 
SELECT last_name,job_title
FROM employees e
INNER JOIN jobs j
ON e.`job_id` = j.`job_id`
WHERE e.`last_name` LIKE '%e%';

（2） Non equivalent connection

# Check the salary level of employees 
SELECT salary,grade_level
FROM employees e
INNER JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`;
# Wages in lowest_sal and highest_sal Between 

（3） Self join

# Check the name of the employee , The name of the superior 
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`=m.`employee_id`;

6.4.2.2 External connection outer

1、 Use scenario ： Used to query a table with , The other table doesn't have records .
2、 characteristic ：
The query result of the external connection is all the records in the main table , If there is a match from the table , The value of the match , If there is no match from the table , Is displayed null.

External connection query results = Internal connection result + Records that exist in the master table but not in the slave table

The test table

# Create two tables 
CREATE TABLE test_1(
	id INT,
	`name` VARCHAR(20),
	girl_id INT
);

CREATE TABLE test_2(
	id INT,
	`name` VARCHAR(20)
);

INSERT INTO test_1(id,name,girl_id)VALUE (1," Liu Ye ",4);
INSERT INTO test_1(id,name,girl_id)VALUE (2," Hu Jun ",9);
INSERT INTO test_1(id,name,girl_id)VALUE (3," Deng Chao ",7);
INSERT INTO test_1(id,name,girl_id)VALUE (4," Liu Yijun ",91);

INSERT INTO test_2(id,name) VALUE (6," Xiaohong ");
INSERT INTO test_2(id,name) VALUE (7," Sun Li ");
INSERT INTO test_2(id,name) VALUE (8," Little black ");
INSERT INTO test_2(id,name) VALUE (2," Little green ");

desc test_1;
desc test_2;
show tables;

SELECT * from test_1;
SELECT * from test_2;

3、 classification ：
（1） Left lateral ：left 【outer】join
left join On the left is the main watch .

#  The left outer join 
SELECT a.girl_id, b.*
FROM test_1 a
LEFT OUTER JOIN test_2 b
ON a.`girl_id` = b.`id`

（2） Right outside ：right 【outer】join
right join The one on the right is from the table .

#  Right connection 
SELECT a.girl_id, b.*
FROM test_1 a
RIGHT OUTER JOIN test_2 b
ON a.`girl_id` = b.`id`

（3） Total external ：full 【outer】join
Table take Union

Left outer and right outer exchange the order of two tables , Can achieve the same effect

6.4.2.3 Cross connect cross

grammar ：

SELECT b.*,bo.*
FROM beauty b
CROSS JOIN boys bo;

7、 Subquery

7.1 meaning

In other statements select sentence , Called subquery or inner query , The external query statement is called primary query or external query

7.2 characteristic

The idea of subquery is , Take the first one. sql Query result of statement , In the second sql Use in statement , At this time, the first one sql Result of statement , In the second sql Can act as a where A value in the condition , Or act as a virtual table .

（1） Subqueries in parentheses
（2） Subqueries are usually placed on the right side of the condition
（3） The execution of the subquery takes precedence over the execution of the main query

7.3 classification

• Where the subquery appears
  （1）where or having Back *（ Scalar subquery 、 Column query 、 Line sub query ）
  （2）select Back （ Only scalar subqueries are supported ）
  （3）from Back （ Support table subquery ）
  （4）exists Back （ Support table subquery ）

where/having Back *

• Scalar subquery （ The result is only one row and one column ）

#  Example 1 ： Inquire about job_id And 141 Same as employee No ,salary Than 143 The name of the employee with more employees ,job_id, And wages 

#  The first (1) Step ： Inquire about 141 Of employee No job_id
SELECT job_id
FROM employees
WHERE employee_id = 141

#  The first (2) Step ： Inquire about 143 The no. salary
SELECT salary
FROM employees
WHERE employee_id=143

#  The first (3) Step ： Check the name of the employee ,job_id, And wages , requirement job_id=(1), also salary>(2)
SELECT last_name,job_id,salary
FROM employees
WHERE job_id=(
	SELECT job_id
	FROM employees
	WHERE employee_id = 141
) AND salary>(
	SELECT salary
	FROM employees
	WHERE employee_id=143
)

------------------------------------------------------
#  Example 2 ： The minimum wage is greater than 50 The Department of minimum wage id And its minimum wage 

#  The first (1) Step ： Inquire about 50 The minimum wage of department No 
SELECT 
	MIN(salary)
FROM 
	employees
WHERE 
	department_id=50

#  The first (2) Step ： Check the minimum wage of each department 
SELECT 
	MIN(salary)
FROM
	employees
GROUP BY
	department_id

#  The first (3) Step ： stay （2） Filter based on ,min(slary)>(1)
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
	SELECT MIN(salary)
	FROM employees
	WHERE department_id=50
)

• Column query （ The result is only one column with multiple rows ）

characteristic ： Back to multiple lines
Use the multiline comparison operator , for example ：IN,NOT IN,ANY,SOME,ALL

#  return location_id yes 1400 or 1700 The names of all employees in the Department 

#  The first (1) Step ： Inquire about location_id yes 1400 or 1700 Department number of 
SELECT department_id
FROM departments
WHERE location_id in (1400,1700)

#  The first (2) Step ： The required department number is (1) One of the list 
SELECT last_name
FROM employees
WHERE department_id in(
	SELECT department_id
	FROM departments
	WHERE location_id in (1400,1700)
)

• Line sub query （ The result is one row with multiple columns or multiple rows with multiple columns ）

#  Query the employee information with the lowest employee number and the highest salary 

#  Search for the minimum employee number 
SELECT MIN(employee_id)
FROM employees

#  Query maximum wage 
SELECT MAX(salary)
FROM employees

#  final result 
SELECT *
FROM employees
WHERE (employee_id,salary)=(
	SELECT MIN(employee_id),MAX(salary)
	FROM employees
)

select Back

#  Query the number of employees in each department 
SELECT d.*,(
	SELECT count(*)
	FROM employees
)
FROM departments d

from Back

#  Query the average wage level of each department 

#  The first (1) Step ： Query the average salary of each department 
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id

#  The first (2) Step ： Connect (1) The result set and job_grades surface , Screening criteria average wage between lowest_sal and highest_sal
SELECT ag_dep.*, g.`grade_level`
FROM(
	SELECT AVG（salary) ag,department_id
	FROM employees
	GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal and highest_sal

exists Back （ Correlation subquery ）

exists The only result is 0 and 1

#  Check the Department name of the employee 
SELECT department_name
FROM departments d
WHERE EXISTS(
	SELECT *
	FROM employees e
	WHERE d.`department_id`=e.`department_id`
)

8、 Paging query

• Application scenarios
  When one page of data to be displayed is incomplete , Paging submission is required sql request

• grammar

select  Query criteria 
from  surface 1
[join type join  surface 2]
on  Connection condition 
where  filter 
group by  Grouping field 
having  Filtering after grouping 
order by  Sorted fields 
limit offset,size;

# offset, To display the starting index of an entry , Index from 0 Start 
# size, Number of items to display 
#  Be careful limit Keywords must be at the end 

• Example

#  Check the first five employee information 
#  Check the first five employee information 
SELECT * 
FROM employees
LIMIT 0,5

The formula
hypothesis ： Number of pages to display ：page, Number of entries per page size
that ：limit (page-1)*size,size

9、 The joint query

Definition

union union , Merge , Combine the results of multiple query statements into one result .

#  Check department number >90 Or the mailbox contains a Employee information 
#  The first way ：OR
SELECT * 
FROM employees
WHERE email
LIKE '%a%'
OR
department_id>90;

#  The second way UNION The joint query 
SELECT * 
FROM employees
WHERE email LIKE '%a%'
UNION
SELECT *
FROM employees
WHERE department_id>90;

Joint query features

1. The number of query columns of multiple query statements is required to be consistent
2. The type and order of each column of a query that requires multiple query statements should be the best
3. UNION Keywords are de duplicated by default , have access to union all Can not go heavy