MySQL Basic query

#进阶1: Basic Query
/*
Grammar:
Select: Query list from table name;

Similar to: System.out.println (something printed);

Characteristics:
1. The query list can be: A field in a table, a constant value, an expression, a function
2, the result of the query is a virtual table
*/
Use MyEmployees

#1, querying a single field in a table
SELECT last_name from Employees;

#2, querying multiple fields in a table
SELECT Last_name,salary,email from Employees;

#3, querying all fields in a table
SELECT
' First_Name ',
' Last_Name ',
' Email ',
' Phone_number ',
' job_id ',
' Salary ',
' Commission_pct ',
' manager_id ',
' HireDate '
From
Employees;

#方式二
SELECT * FROM Employees

#查询常量值
SELECT 100;
SELECT ' John ';

#查询表达式
SELECT 100%98;

#查询函数
SELECT VERSION ();

#7, aliases
/*
1. Easy to understand
2. If the field you want to query has the same name, use an alias to differentiate
*/
#方式一: Using as
SELECT 100%98 as results;
SELECT Last_Name as surname, first_name as name from employees;
#方式二: Use spaces
SELECT last_name Surname, first_name name from employees;

#案例: Query salary, display results as Out_put
Select Salary "Out_put" from employees;

#8, go heavy

#案例: Query All department numbers involved in the employee table
SELECT DISTINCT department_id from employees;

The function of #9 and + number
/*
The + sign in Java
1, Operator: Two operands are numeric
2, connector: As long as there is one operand is a string
MySQL in the + number
There is only one function: operator
Select 100+90; two operands are numeric, then add operation
Select ' 123 ' +90; one of them is a character type, attempting to convert a character value into a numeric type
If the conversion succeeds, continue with the addition operation
Select ' John ' +90; convert the character to 0 if the conversion fails

Select Null+10; As long as one of the two is null, the result must be null

*/

#案例: Query employee name and last name concatenated into a field and displayed as name
SELECT CONCAT (' A ', ' B ', ' C ') as result;
SELECT CONCAT (' last_name ', ' first_name ') as name from employees;

MySQL Basic query