Review of MySQL (3): query operation

MySq Review ( 3、 ... and )： Query operation

Grammar format ：（ Any one sql All statements are based on “;” At the end of the ）

select Field name 1, Field name 2, Field name 3,…from Table name ;

Simple query statements

Query a field

• Check the name of the employee

  select ename frome emp;
  

select Is the key word ,select And field names are separated by spaces 1,from Represents the table to query , It is separated from the field by a space .

Query multiple fields

• Query employee's number and name

  select empno,enmae from emmp;
  

  Be careful ： Query multiple fields ,select The fields in are separated by commas , The last field is in from The fields before cannot use commas .

Query all fields

You can put all the fields in select After statement , But if there are too many fields , inconvenient . You can use the following convenient ways to query all fields .

select * from Table name ;

select * from  emp;

Calculate the employee's annual salary

• List the employee's number , Name and annual salary

  select empno,ename,sal*12 from emp;
  

select You can use operators in statements , But there are some problems , The field name of annual salary is not accurate . Use as Keyword rename the fields of the table .

• Display the queried fields as user-defined names or Chinese

   select empno ' Employee number ',ename as " Employee name ",sal as ' Annual salary ' from emp;
  --  Be careful : Although in cmd Windows can be without double or single quotation marks , But in the later standard format, the string must be added with English single quotation marks  |  Double quotes , among as You can omit it 
  

  

Conditions of the query

Conditional query needs to use where sentence ,where Must be on from After the statement table .

The following operators are supported ：

Operator	explain
=	be equal to
<> or !=	It's not equal to
<	Less than
<=	Less than or equal to
>	Greater than
>=	Greater than or equal to
between … and ….	Between two values , Equate to >= and <=
is null	by null（is not null Not empty ）
and	also
or	perhaps
in	contain , It's equivalent to more than one or（not in Not in this range ）
not	not You can take non , Mainly used in is or in in
like	like It is called fuzzy query , Support % Or underline match % Match any character underscore , An underscore matches only one character

The equal sign operator

• The inquiry salary is 500 The employees'

  select empno,enmae,sal from empp where sal = 5000;
  

• Inquire about job by MANAGE The employees'

  select empno,ename from emp where job = "manager";
  --  because job For the string , So add double or single quotation marks 
  select empno,ename from emp where job = 'manager';
  -- mysql The statements of are case insensitive by default , So it can also be written as the following 
  select empno,ename from emp where job = "MANAQER";
  

<> The operator

• Inquiring about salary is not equal to 5000 The employees'

  select empno,ename,sal from emp where sal <> 5000;
  

• It can also be written in the following way , But the first one

  select empno,ename,sal from emp where sal != 5000;
  

• Querying a job position is not equal to MANAGER The employees'

  select empno, ename from emp where job <> 'MANAGER';
  --  Again , Double quotation marks or single quotation marks should be added to the string 
  

between…and… The operator

• Check the salary in 1600 To 3000 The employees'

1. use >= and <=

   select empno,ename,sal from emp where sal >= 1600 and sal <= 3000;
   

2. use between…and…

   select empno, ename, sal from emp where sal between 1600 and 3000;
   

is null Operator

• null It's empty , But it's not an empty string , by null You can set this field to be left blank , If the query is null Field of , use is null

• Query the employee whose allowance is empty

  --  because null type 1 A special , You have to use is To compare 
  select * from emp where comm is null;
  

  

  because null A special , use = There is no result in comparison .

  

AND Operator

and The meaning of and , All conditions must be met

• The job position is MANAGER, Salary greater than 2500 The employees'

  select * from emp where job = "MANAGER" and sal > 2500;
  

OR Operator

or As long as the conditions are met , It is equivalent to containing

• Query out job by manager perhaps job by salesman The employees'

  select * from emp where job = 'MANAGER' or job = 'SALESMAN';
  

The priority of the expression

• Query salary greater than 1800, And the department code is 20 perhaps 30 People who

1. Wrong writing

   select * from emp where sal > 1800 and deptno = 20 or deptno = 30;
   

   As a result, the salary will be less than 1800 The data of , The reason is the priority of the expression , Filter first sal > 1800 and deptno = 20, And then deptno = 30 The staff merged , So it's not right

2. Write it correctly

   Try to use parentheses ！

   select * from emp where sal > 1800 and (deptno = 20 or deptno = 30);
   

IN Operator

in Means to include , It can be used or Express , however in It will be more concise .

• Find out job by manager perhaps job by salesman The employees'

select * from emp where job in ('manager','salesman');

• Find out the salary is 1600 perhaps 3000 The employees'

select * from emp where sal in (1600,3000);

NOT Operator

• Find out that the salary does not include 1600 And does not contain 3000 The employees'

1. The first way to write it

select * from emp where sal <> 1600 and sal <> 3000;

2. The second way

select * from emp where not (sal = 1600 or sal = 3000);

3. The third way

select * from emp where sal not in (1600,3000);

• Find out that the allowance is not null All employees

select * from emp where sal is not null;

LIKE Operator

• like Fuzzy query can be realized ,like Support % Match the underline
• Check the name with M Employees at the beginning

select * from emp  where ename like 'M%';

• Check the name with N All employees at the end

select * from emp where ename like '%N';

• The query name contains O All employees

select * from emp where enmae like '%O%';

• The second character in the query name is A All employees

select * from emp where ename like '_A%';

Like in % and _ The difference between ？

% Matches the number of occurrences of any character

The underscore can only match one character

Like Expressions in must be enclosed in single or double quotation marks ！