Basic select statement

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【MySQL】

1. Query constant calculation

grammar ：

SELECT  Constant operation expression 1,  Constant operation expression 2, ...
[ FROM dual ];

SELECT 1+1, 3*2;

Query constants do not need to be looked up from the table , have access to dual ( False watch ) Complete the structure of the query statement .

SELECT 1+1, 3*2
FROM dual;

2. The basic syntax of a query statement

The basic syntax of a query statement ：

SELECT  Field 1,  Field 2, ... 
FROM  Table name ;

2.1 Query all fields

grammar ：

SELECT * 
FROM  Table name ;

• * Represents all fields in the table .

Inquire about employees All fields in the table ：

SELECT * 
FROM employees;

In general , Unless you need to use all the field data in the table , Otherwise, it is best not to use wildcards * .

Using wildcards can save Enter the query statement Time for , However, getting unwanted column data often reduces the efficiency of queries and applications used .

The advantage of wildcards is , When you don't know the name of the desired column , You can get them through wildcards .

In production environment , Direct use is not recommended SELECT * The query .

2.2 Query the specified field

grammar ：

SELECT  Field 1,  Field 2, ... 
FROM  Table name ;

Specify the fields in the query table , You need to ensure that fields exist in the table , At the same time, make sure that the field name is spelled correctly .

Inquire about employees In the table employee_id、last_name、salary Field ：

SELECT employee_id, last_name, salary 
FROM employees;

3. Alias of column

When inquiring , You need to rename the queried fields ( Take the alias ), Keywords can be used AS (alias – Alias ).

Inquire about employees In the table employee_id、last_name、salary Field ：

SELECT employee_id AS id, 
       last_name AS lname, 
       salary AS sal 
FROM employees;

AS Keywords can be omitted

SELECT employee_id id, 
	   last_name lname, 
	   salary sal 
FROM employees;

Field aliases can use a pair Double quotes Lead up , Don't use single quotes ( because MySQL The grammar is not rigorous , stay MySQL You can't make a mistake , However, errors will be reported in some databases ).

Add ： Strings are enclosed in single quotes , Don't use double quotes , Will report a mistake .

SELECT employee_id AS "id", 
	   last_name AS "lname", 
	   salary AS "sal" 
FROM employees;

The query results of the three statements are the same ：

The alias of the field is enclosed in double quotation marks , In order to Include spaces or special characters in the alias and be case sensitive , If there is a space in the alias, be sure to wrap it in double quotation marks .

SELECT employee_id emp_id,
	   last_name AS lname,
	   department_id " department id",
	   salary * 12 AS "annual sal" 
FROM employees;

4. Remove duplicate lines

By default , The query will return all rows , Include repeating lines .

Query which departments are in the employee table id：
No weight loss ：

SELECT department_id 
FROM employees;

De duplication ( Use keywords DISTINCT )：

SELECT DISTINCT department_id 
FROM employees;

SELECT salary,DISTINCT department_id
FROM employees;

salary If there is no weight removal, it will all be displayed , DISTINCT department_id It's going to be weightless , The two clashed , Will report a mistake .

[42000][1064] You have an error in your SQL syntax; 
check the manual that corresponds to your MySQL 
server version for the right syntax to use 
near 'DISTINCT department_id FROM employees' 
at line 1

Yes department_id, salary Repeat and de duplicate the query results of both ：

SELECT DISTINCT department_id, salary 
FROM employees;

Only when department_id, salary Only records with the same values of the two fields will be de duplicated .

DISTINCT In fact, it is to de duplicate the combination of all the following column names .
The end result is 74 strip , Because of this 74 Departments in results id And salary There is no repetition after combination . If you want to see the different departments （department_id）, You just need to write DISTINCT department_id that will do , There is no need to add other column names after .

5. Null values participate in the operation

Null value ：null

stay MySQL in , A null value is not equal to an empty string . An empty string is of length 0 String , A null value means that there is no specific value .

null Not equal to 0 ( Values for 0, Not empty ),''( The length is 0 String , Not empty ),'null'( character string , Not empty ).

SELECT * 
FROM employees;

Null values participate in the operation , The result must also be empty .

SELECT employee_id, salary AS " Monthly wages ", 
	   salary*(1+commission_pct)*12 AS " Annual wage ", 
	   commission_pct 
FROM employees;

Because some records commission_pct The median value is empty , So the corresponding calculation of this record “ Annual wage ” Also empty .

Solutions to practical problems ： introduce IFNULL()

SELECT
    employee_id,
    salary AS " Monthly wages ",
    #  If  commission_pct  It's empty , Use  0  To replace 
    salary*(1+IFNULL(commission_pct, 0))*12 AS " Annual wage ",
    commission_pct
FROM employees;

6. mark of emphasis

mark of emphasis ：` ( The top left corner of the keyboard ,1 side )

When the table name or field name is the same as the keyword , You need to use a bullet to quote the table name or field name .

SELECT * 
FROM ORDER;

Report errors

[42000][1064] You have an error in your SQL syntax; 
check the manual that corresponds to your MySQL server 
version for the right syntax to use near 'ORDER' at line 1

SELECT * 
FROM `order`;

We need to ensure that the fields in the table 、 Table names, etc. have no and reserved words 、 Database systems or common methods conflict . If it's really the same , stay SQL A pair of... Is used in the statement `（ mark of emphasis ） Lead up .

7. Query constant

Constants will be spliced with each row of the query result .

SELECT
    'hello',
    123,
    employee_id,
    last_name
FROM employees;

8. Display table structure

grammar ：

DESCRIBE  Table name ;

Abbreviation ：

DESC  Table name ;

Show employees The structure of the table ：

DESCRIBE employees;

DESC employees;

Query results , Shows the details of the fields in the table .

• among , The meanings of each field are explained as follows ：
  • (1) Field： Field name .
  • (2) Type： Represents the field type
  • (3) Null： Indicates whether the column can store NULL value .
  • (4) Key： Indicates whether the column is indexed .PRI Indicates that the column is part of the table primary key ;UNI Indicates that the column is UNIQUE Part of index ;MUL Indicates that a given value is allowed to appear more than once in a column .
  • (5) Default： Indicates whether the column has a default value , If there is , So what's the value .
  • (6) Extra： Represents additional information about a given column that can be obtained , for example AUTO_INCREMENT etc. .

9. Filtering data

Use WHERE Words and expressions , Filter out rows that do not meet the criteria ,WHERE Clause in the wake of FROM Clause , The filter condition is declared in FROM Behind the structure .

grammar ：

SELECT  Field 1,  Field 2, ...
FROM  Table name 
WHERE  Filter conditions ;

Inquire about 90 Employee information of department No ：

SELECT *
FROM employees
WHERE department_id=90;

Inquire about last_name by ‘King’ Employee information ：

SELECT *
FROM employees
WHERE last_name='King';

SELECT *
FROM employees
WHERE last_name='king';

The query result of this statement is the same as the above
reason ： stay windows In the environment , Case insensitivity leads to MySQL Is not case sensitive , But because of MySQL The imprecision of causes the case in the string to be indistinguishable , So the results of the above two sentences are the same .
This will not happen in other data .

stay windows Environment is not size sensitive , Refers to the field name , Table name , keyword , Exclude strings within quotation marks .

10. Basic SELECT Sentence practice

【 subject 】

# 1. Query employees 12 The sum of monthly wages , It's also called ANNUAL SALARY

# 2. Inquire about employees Remove duplicates from the table job_id Future data  

# 3. Query salary greater than 12000 The name and salary of the employee  

# 4. The employee number is 176 The name and department number of the employee  

# 5. Display table  departments  Structure , And query all the data 

【 answer 】

1. Query employees 12 The sum of monthly wages , It's also called ANNUAL SALARY

#  understand  1 ： Calculation 12 Monthly basic salary 
SELECT
    employee_id,
    last_name,
    salary*12 AS "ANNUAL SALARY"
FROM employees;

#  understand  2 ： Calculation 12 Monthly basic salary and bonus 
SELECT
    employee_id,
    last_name,
    salary*12*(1+IFNULL(commission_pct,0)) AS "ANNUAL SALARY"
FROM employees;

2. Inquire about employees Remove duplicates from the table job_id Future data

SELECT DISTINCT job_id
FROM employees;

3. Query salary greater than 12000 The name and salary of the employee

SELECT
    last_name,
    salary
FROM employees
WHERE salary>12000;

4. The employee number is 176 The name and department number of the employee

SELECT
    last_name,
    department_id
FROM employees
WHERE employee_id=176;

5. Display table departments Structure , And query all the data

DESC departments;
DESCRIBE departments;

SELECT *
FROM departments;