Database_01_Basic query

#Advanced 1: Basic query
/* Syntax
select query list from table name;
Features:
1. The query list can be: fields in the table, constant values, expressions, functions
2. The result of the query is a virtual table
*/
#0. Use the database

 USE employees;

#1. Query a single field in the table

SELECT last_name FROM employees;

#2. Query multiple fields in the table

SELECT last_name,salary,email FROM employees;

#3. Query all fields in the table

SELECT * FROM employees;

#4. Query constant value

SELECT 100;

#5. Query expression

SELECT 100*99;

#6. Query function

SELECT VERSION();

#7.起 alias
# Method 1:

SELECT 
     last_name AS 姓,first_name AS 名 
FROM employees; 

#法二:

SELECT last_name 姓,first_name 名 FROM employees; 

#Case: query salary, the display result is out put;

SELECT salary AS "out put" FROM employees;

#8.去重
#Query all department numbers involved in the employee table

SELECT DISTINCT department_id FROM employees;

#9. The effect of the + sign
/*
select 100+90; If both operands are of numeric type, do an addition operation
select '123'+90; one of them is a character type, trying to convert a character type value into a numeric type
conversion If it succeeds, do the addition operation
select'john'+90; if the conversion fails, the character type is converted to 0
select null+10; if one party is NULL, the result is NULL
*/
#Case: query the employee's name and surname into one field, and Display as name

SELECT 
      CONCAT(last_name,first_name) 
AS 
      姓名 
FROM 
      employees;

#after class homework

SELECT 
   CONCAT(`last_name`,',',`phone_number`,',',`job_id`,',',`salary`) 
AS 
   out_put FROM employees;

SELECT 
   IFNULL(commission_pct,0) AS 奖金率,
   commission_pct
FROM
   employees;