* # DQL language

Display table structure desc table;

Query constant values

select 100;
select 'python';
Copy the code

Query expression (four operations)

select 100*98;
Copy the code

Query function

seelct version();
Copy the code

names

/* Easy to understand if the fields to be queried have the same name, alias can distinguish */

Methods a

Select 100*98 as; Select first_name as surname,last_name as name from employees;Copy the code

Way 2

Select 100*98; Select first_name,last_name from empl0yees;Copy the code
SQL > select * from salary; SQL > select * from salary;
select salary as 'out put' from employees;
Copy the code

duplicate removal

Example: Query all department numbers involved in the employee table
select distinct department_id from employees;
Copy the code
Example of the function of ‘+’ : Query the first and last names of employees and join them as one field

/ *

Mysql uses’ + ‘as the operator

Both numbers are of numerical type

If one of the characters is a character, the character is converted to a numeric value to continue the operation

If the conversion fails, the character is converted to 0

If either party is null, the result must be NULL

* /

Splicing functions:
concat(str1,str2...) Select concat(first_name,last_name) as name from employees;Copy the code

Conditions of the query

Select * from table where (select * from table);Copy the code

1. Conditional expression filtering:

Conditional operatorCopy the code

3. Logical expression filtering

Logical operators: && | |! Or "and or not.Copy the code

3, fuzzy query

Like between in is NULL Safely equal to: <=> You can identify null values and common valuesCopy the code

The sorting

Select * from table where order by sort asc\descCopy the code

Multifield sort

Select * from table where order by asc\desc, asc\descCopy the code

Order by: order by: order by: order by: order by: order by

DML (add delete change) #DDL #TCL