Basic operations and common functions of MySQL table creation

Catalog

Database command

Create tables and constraints

mysql Of comment How to use it?

Explain cascading deletion And modify

mySql Common functions

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Database command

establish
Command format
create database Database name
create database if not exists Database name default charset utf8 collate utf8_general_cs;
The default database encoding set ：utf8
 collate Indicates that the sorting rule is utf8_general_ci
Common sort types
utf8_general_cs（ Case sensitive ）
utf8_genera_ci（ Case insensitive ）
see
show databases
Delete
drop database Database name
Dangerous operation , Backup first

Create tables and constraints

    Build table
         Command format
  

     CREATE TABLE  The name of the table (
         Column name 1  data type  NOT NULL,
         Column name 2  data type ,
         Column name 3  data type ,
        unique( Column name 1[, Column name 2,..., Column name N])
    )
      Example 
 create table t_student
                 (
   sid    int not null comment ' Student number ',
   sname  varchar(60) not null comment ' full name ',
   sex    tinyint not null default 1 comment ' Gender ：1 male , 2 Woman ',
   age    tinyint not null comment '  Age ',
   icard  varchar(18) not null comment ' Id card , Unique constraint ',
   primary key (sid),
   unique key AK_Key_2 (icard)
) comment ' Student information sheet ';

mysql Of comment How to use it?

stay MySQL In the database , The annotation of a field or column is in the form of an attribute comment To add .

In the script to create a new table ,

You can add... In the field definition script comment Property to add comments .

The sample code is as follows ：

1 2 3

create table test(     id int not null default0 comment ' user id' )

If it's a built table ,
You can also use the command to modify the field , And then add comment Attribute definitions , You can add comments .
The sample code is as follows ：

1 2	alter table test change column id id int not null default0 comment ' The test table id'

View the comments of all fields in the existing table ？

You can use the command ：show full columns from tablename Check it out. ,

Examples are as follows ：

1	show full columns from test;

1、 Write comments when creating tables

1 2 3 4

create table test1 (     field_name int comment ' Field comments ' )comment=' Notes to table ';

2、 Modify the notes of the table

1	alter table test1 comment ' Comments on the modified table ';

3、 Modify the comment of the field

1 2	alter table test1 modify column field_name int comment ' Modified field comments '; -- Be careful ： The field name and field type are just as they are

4、 How to view table comments

1 2 3 4 5

-- In the generated SQL Look in the sentence show  create  table  test1; -- Look in the metadata table useinformation_schema; select * from TABLES where TABLE_SCHEMA='my_db'andTABLE_NAME='test1'\G

5、 How to view field comments

1 2 3 4

--show show  full  columns  from  test1; -- Look in the metadata table select * from COLUMNS where TABLE_SCHEMA='my_db'andTAB

  constraint
         Primary key constraint
 PRIMARY KEY
         Add primary key
ALTER TABLE Table name ADD PRIMARY KEY( Primary key name )
        Delete primary key
ALTER TABLE Table name DROP PRIMARY KEY
         Non empty constraint
sid      INT NOT NULL COMMENT ' Student number ';

With the above t_student Establish primary and foreign key relationships

CREATE TABLE t_score
(
   id                   INT NOT NULL COMMENT ' Record serial number ',
   sid                  INT NOT NULL COMMENT ' Student number ',
   cid                  INT NOT NULL COMMENT ' Course ID',
   score                FLOAT COMMENT ' achievement ',
   PRIMARY KEY (id),
   FOREIGN KEY (sid) REFERENCES t_student (sid) ON DELETE RESTRICT ON UPDATE RESTRICT,
   UNIQUE KEY AK_Key_2 (sid, cid)
);

explain ： sid Is the foreign key of this table , relation t_student In the table sid Primary key ,ON DELETE RESTRICT ON UPDATE RESTRICT Explain that if there is data in this table , The associated key of the main table cannot be deleted or updated .

Explain cascading deletion And modify

A Tabular id yes B Foreign key

One 、on delete

1. on delete cascade： Delete A Table data when , Corresponding B The data of the table will also be deleted

2. on delete set null： Delete A Table data when , Corresponding B The foreign key field of the table is empty （ The premise is that this field can be empty ）

If you don't add the above content when adding foreign keys , Delete A Table data when , If the corresponding B If there is data in the table, an error will be reported

Two 、on update

1. on update cascade： to update A Table data id when , Corresponding B The foreign key fields of the table will also be updated

2. on update set null： to update A Table data id when , Corresponding B The foreign key field of the table is empty （ The premise is that this field can be empty ）

If you don't add the above content when adding foreign keys , to update A Tabular id, If the corresponding B If there is data in the table, an error will be reported

Add foreign key statement ：alter table user add constraint FK_1 foreign key(role_id) references role(id) on delete cascade on update cascade;
         

Foreign key constraints
     Add foreign key
ALTER TABLE Table name ADD FOREIGN KEY( Name of the foreign key ) REFERENCES Main table name ( Primary key name )
     Delete foreign key
ALTER TABLE Table name DROP FOREIGN KEY Constraint name

Unique constraint
UNIQUE KEY Constraint name ( Field )
   Create a unique constraint
ALTER TABLE Table name ADD UNIQUE( Column name 1[, Column name 2,..])
create unique index UserNameIndex on 't_user' ('username')
     Delete unique constraint
ALTER TABLE Table name DROP INDEX Unique constraint index name

Default constraint
  default 

mySql Common functions

（ One ） Number function
1、ABS(x) return x The absolute value of

SELECT ABS(-1);
----  return 1

2、AVG(price) Returns the average value of an expression ,price It's a field

SELECT AVG(age) FROM student;

  3、CEIL(x)/CEILING(x) Returns greater than or equal to x Minimum integer of （ Rounding down ）

SELECT CEIL(1.5);
SELECT CEILING(1.5);   
----  return 2

4、FLOOR(x) Returns less than or equal to x Maximum integer for （ Rounding down ）

SELECT FLOOR(1.5); 
----  return 1

5、EXP(x) return e Of x Power

SELECT EXP(3);
----  Calculation  e  The third power of , return 20.085536923188

6、GREATEST(expr1, expr2, expr3, …) Returns the maximum value in the list

SELECT GREATEST(3, 12, 34, 8, 25);
----  Returns the maximum value in the following list of numbers 34

7、LEAST(expr1, expr2, expr3, …) Returns the minimum value in the list

SELECT LEAST(3, 12, 34, 8, 25);
----  Returns the minimum value in the following list of numbers 3

8、LN Returns the natural logarithm of a number

SELECT LN(2);
----  return  2  The natural logarithm of ：0.6931471805599453

9、LOG(x) Return to the natural logarithm ( With e Log base )

SELECT LOG(20.085536923188);
----  return  3

10、MAX(expression) Return field expression Maximum of

SELECT MAX(age) AS maxAge FROM Student;
---- age Maximum 

11、MIN(expression) Return field expression Maximum of

SELECT MIN(age) AS minAge FROM Student;
---- age minimum value 

12、POW(x,y)/POWER(x,y) return x Of y Power

SELECT POW(2,3);
SELECT POWER(2,3);
----  return 2  Of  3  Power ：8

13、RAND() return 0 To 1 The random number

SELECT RAND();
----  return  0  To  1  The random number , if () There are numbers in it ,RAND(x),x Phase at the same time , Same return value 

14、ROUND(x) Return from x The nearest integer （ rounding ）

SELECT ROUND(1.23456);
----  return  1

15、SIGN(x) return x The symbol of ,x It's a negative number 、0、 Positive numbers return respectively -1、0 and 1

SELECT SIGN(-10);
----  return  -1

16、SQRT(x) return x The square root of

SELECT SQRT(25);
----  return 5

17、SUM(expression) Returns the sum of the specified fields

SELECT SUM(age) AS totalAage FROM Student;
----  return age The sum of 

18、TRUNCATE(x,y) Returns the value x Keep it after the decimal point y The value of a （ And ROUND The biggest difference is that there is no rounding ）

SELECT TRUNCATE(1.23456,3);
----  return 1.234

（ Two ） String function
1、 Return string s Of the first character of ASCII code

SELECT ASCII('AB');
----  return A Of ASCII Code value ：65

2、LENGTH/CHAR_LENGTH(s)/CHARACTER_LENGTH(s) Return string s The number of characters

SELECT LENGTH('1234');
----  return 4

3、CONCAT(s1,s2…sn) character string s1,s2 When multiple strings are merged into a single string

SELECT CONCAT('hel','llo');
----  return hello

4、FIND_IN_SET(s1,s2) Return in string s2 China and s1 The location of the matching string

SELECT FIND_IN_SET("c", "a,b,c,d,e");
----  return 3

5、FORMAT(x,n) The function can change the number to x format “#,###.##”, take x Keep it after the decimal point n position , Round the last one

SELECT FORMAT(250500.5634, 2); 
----  return 250,500.56

6、INSERT(s1,x,len,s2) character string s2 Replace s1 Of x The starting length of the position is len String

SELECT INSERT("google.com", 1, 6, "runnob");
----  return runoob.com

7、LOCATE(s1,s) From a string s In order to get s1 The beginning of

SELECT LOCATE('st','myteststring');
----  return 5

8、LCASE(s)/LOWER(s) The string s All of the letters of the are changed into lower case letters

SELECT LOWER('RUNOOB');
----  return runoob

9、UCASE(s)/UPPER(s) The string s All of the letters of the alphabet become capital letters

SELECT UCASE('runoob');
----  return RUNOOB

10、TRIM(s) Remove string s The spaces at the beginning and the end

SELECT TRIM('    RUNOOB    ');
----  return RUNOOB

11、LTRIM(s) Remove string s The space at the beginning

SELECT LTRIM('    RUNOOB    ');
----  return  ’RUNOOB   ‘

12、RTRIM(s) Remove string s Space at the end

SELECT RTRIM('    RUNOOB    ');
----  return  ’    RUNOOB‘

13、SUBSTR(s, start, length) From a string s Of start The position intercept length is length Substring of

SELECT SUBSTR("RUNOOB", 2, 3) AS ExtractString;
----  From a string  RUNOOB  No  2  Location intercepts  3 individual   character , return UNO

14、SUBSTR/SUBSTRING(s, start, length) From a string s Of start The position intercept length is length Substring of

SELECT SUBSTR/SUBSTRING("RUNOOB", 2, 3);
----  From a string  RUNOOB  No  2  Location intercepts  3 individual   character , return UNO

15、POSITION(s1 IN s) From a string s In order to get s1 The beginning of

SELECT POSITION('b' in 'abc');
----  return 2

16、REPEAT(s,n) The string s repeat n Time

SELECT REPEAT('runoob',3);
----  return runoobrunoobrunoob

17、REVERSE(s) The string s In reverse order

SELECT REVERSE('abc');
----  return cba

18、STRCMP(s1,s2) Compare strings s1 and s2, If s1 And s2 Equal return 0 , If s1>s2 return 1, If s1<s2 return -1

SELECT STRCMP("runoob", "runoob");
----  return 0

（ 3、 ... and ） Date function
1、CURDATE()/CURRENT_DATE() Return current date

SELECT CURDATE();
SELECT CURRENT_DATE();
----  return 2019-02-19

2、CURRENT_TIME()/CURTIME() Return current time

SELECT CURRENT_TIME();
----  return 11:40:45

3、CURRENT_TIMESTAMP() Returns the current date and time

SELECT CURRENT_TIMESTAMP();
----  return 2019-02-19 11:41:32

4、ADDDATE(d,n) Calculate start date d add n The date of day

SELECT ADDDATE("2017-06-15", INTERVAL 10 DAY);
----  return 2017-06-25

5、ADDTIME(t,n) Time t add n The second time

SELECT ADDTIME('2011-11-11 11:11:11', 5);
----  return 2011-11-11 11:11:16

6、DATE() Extract a date value from a date or datetime expression

SELECT DATE("2017-06-15 11:11:16");
----  return 2017-06-15

7、DAY(d) Returns the date value d The date part of

SELECT DAY("2017-06-15"); 
----  return 15

8、DATEDIFF(d1,d2) Calculate the date d1->d2 The days between

SELECT DATEDIFF('2001-01-01','2001-02-02'); 
----  return -32

9、DATE_FORMAT By expression f To display the date d

SELECT DATE_FORMAT('2011.11.11 11:11:11','%Y-%m-%d %r');
----  return 2011-11-11 11:11:11 AM

10、DAYNAME(d) Return date d What day is , Such as Monday,Tuesday

SELECT DAYNAME('2011-11-11 11:11:11');
----  return Friday

11、DAYOFMONTH(d) Calculate the date d It's the day of the month

SELECT DAYOFMONTH('2011-11-11 11:11:11');
----  return 11

12、DAYOFWEEK(d) date d What day is today ,1 Sunday ,2 Monday , And so on

SELECT DAYOFWEEK('2011-11-11 11:11:11');
----  return 6

13、DAYOFYEAR(d) Calculate the date d It's the day of the year

SELECT DAYOFYEAR('2011-11-11 11:11:11');
----  return 315

14、EXTRACT(type FROM d) From the date d Get the specified value from the ,type Specifies the value to be returned
type It can be taken as ：
MICROSECOND
SECOND
MINUTE
HOUR
DAY
WEEK
MONTH
QUARTER
YEAR
SECOND_MICROSECOND
MINUTE_MICROSECOND
MINUTE_SECOND
HOUR_MICROSECOND
HOUR_SECOND
HOUR_MINUTE
DAY_MICROSECOND
DAY_SECOND
DAY_MINUTE
DAY_HOUR
YEAR_MONTH

SELECT EXTRACT(MINUTE FROM '2011-12-13 14:15:16');
----  return 15

15、DAYOFWEEK(d) date d What day is today ,1 Sunday ,2 Monday , And so on

SELECT DAYOFWEEK('2011-11-11 11:11:11');
----  return 6

16、UNIX_TIMESTAMP() Get timestamp

SELECT UNIX_TIMESTAMP('2019-2-19');
SELECT UNIX_TIMESTAMP(expression);
----  return 1550505600

17、FROM_UNIXTIME() Time stamp date

SELECT FROM_UNIXTIME(1550505600);
----  return 2019-02-19 00:00:00
SELECT FROM_UNIXTIME(1550505600, '%Y-%m-%d');
----  return 2019-02-19

（ Four ）MySQL Advanced functions
1、IF(expr,v1,v2) If the expression expr establish , Return results v1; otherwise , Return results v2

SELECT IF(1>0,'yes','no');
----  return yes

2、CONV(x,f1,f2) return f1 The base number becomes f2 Hexadecimal number

SELECT CONV(13,10,2);
----  return 1101

3、CURRENT_USER()/SESSION_USER()/SYSTEM_USER()/USER() Return to current user
4、DATABASE() Returns the current database name
5、VERSION() Return the version number of the database