SQL language learning record I

sql The classification of languages
DDL（ Data definition language ）：CREATE\ALTER\DROP\RENAME\TURNCATE
DML（ Data operation language ）;INSERT\DELETE\UPDATE\SELECT
DCL（ Control language ）：COMMIT\ROLLBACK\SAVEPOINT\GRANT\REVOKE

sql Rules and norms of language ：
The rules ：

1\ Three ways of annotation

/* */ Multiline comment
– notes
"#" Also a note

2、 Import an existing data table
1.source File path
2. Graphical interface

show databases Display database
source route
use atguigudb;
show tables

There are three ways to alias columns

SELECT employ_id emp_id FROM employee
Add sa
quotation marks

DISTINCT

select distinct department_id from employee
Remove duplicate data

Null operation

The result of null value participating in the operation is also null Therefore, a null value is not “null” Nor is it. 0
SELECT employee_id,salary,salary * IFNULL(( 1 + commission_pct),0) * 12 as Annual wage FROM employees;

mark of emphasis

If it indicates that the name is the same as the keyword, you need to add “ `` ” To restore normal operation

DESCRIVE

Display table structure describe employees;
DESC employees;

Filter conditions

Inquire about 90 Employee information of department No
select * from employeee where name = ‘king’
Single quotation marks mysql Case sensitive ,ocrale Case sensitive

• Operator

+ - * / div % mod

• SELECT 100 + ‘1’ FROM DUAL; We found out it was 101
  java The result is 1001,sql Middle means addition operation （ Implicit conversion ）
• SELECT 100 + ‘a’ FROM DUAL; At this time will be a regard as 0 To deal with it

Comparison operator

<=> Safety is equal to
<> It's not equal to
character string ‘a’ Be regarded as 0 Value
If you compare a pure string with a string, you won't implicitly convert a value
As long as there is null The result of participation is null, If the value to be queried is null Data should use security equal to

IS NULL \IS NOT NULL \ ISNULL

WHERE ISNULL(commission_pct);
where comminssion_pct <=> null
where not commission_pct <=> null;

least \greatest

in

SELECT department_id ,last_name from employees WHERE department_id in (10,20,30);
The query department number is 10,20,30 People's information

like

SELECT last_name FROM employees where last_name like ‘%a%’;
The percent sign represents an indefinite number of characters （0 individual 1 More than one ）
SELECT last_name FROM employees where last_name like ‘a%’;
With a Start character
Contains characters a And contains characters e Of
SELECT last_name FROM employees where last_name like ‘%a%e%’ or last_name like “%e%a%”;
contain e And contains a The data of

• _ The second character is a
  SELECT last_name FROM employees where last_name like ‘__a%’;

Logical operators

or || and
And && or
and Has a higher priority than or The priority of the
Priority can be perfect without adding parentheses

The sorting operation

If there is no sorting, it is implemented in the order of addition by default
Use orderby Sort the order found
Ascending ：asc By default, it is arranged in ascending order from low to high
Descending ：desc
SELECT employee_id,salary from employees ORDER BY salary desc;
Column aliases can only be used in orderby Cannot be used in where Use in

Pagination

SELECT * FROM employees
LIMIT 0,20
Paged display from 0 Start the first 20 Data
Display the data on the second page
limit 20,20