MySQL foundation 05 DML language

The meaning of database ： data storage 、 Data management
Data management method of database ：

• adopt SQLyog And other management tools to manage database data
• adopt DML Statement management database data

DML Language ： Data operation language

• Used to manipulate data contained in database objects

Include :

• INSERT ( Add data statement )

• UPDATE ( Update data statement )

• DELETE ( Delete data statement )

INSERT command

INSERT INTO Table name [( Field 1, Field 2, Field 3,…)] VALUES(‘ value 1’,‘ value 2’,‘ value 3’)

Be careful :

• Fields or values are separated by commas .
• ’ Field 1, Field 2…’ This part can be omitted , But the added value must be consistent with the table structure , Data columns , The order corresponds to , And the quantity is the same .
• Multiple data can be inserted at the same time , values Separated by commas .
  – How to create a foreign key : Create sub table and foreign key

--  Grade table  (id\ Grade name )
CREATE TABLE `grade` (
`gradeid` INT(10) NOT NULL AUTO_INCREMENT COMMENT ' grade ID',
`gradename` VARCHAR(50) NOT NULL COMMENT ' Grade name ',
PRIMARY KEY (`gradeid`)
) ENGINE=INNODB DEFAULT CHARSET=utf8

--  Student information sheet  ( Student number , full name , Gender , grade , mobile phone , Address , Date of birth , mailbox , ID number )
CREATE TABLE `student` (
`studentno` INT(4) NOT NULL COMMENT ' Student number ',
`studentname` VARCHAR(20) NOT NULL DEFAULT ' anonymous ' COMMENT ' full name ',
`sex` TINYINT(1) DEFAULT '1' COMMENT ' Gender ',
`gradeid` INT(10) DEFAULT NULL COMMENT ' grade ',
`phoneNum` VARCHAR(50) NOT NULL COMMENT ' mobile phone ',
`address` VARCHAR(255) DEFAULT NULL COMMENT ' Address ',
`borndate` DATETIME DEFAULT NULL COMMENT ' Birthday ',
`email` VARCHAR(50) DEFAULT NULL COMMENT ' mailbox ',
`idCard` VARCHAR(18) DEFAULT NULL COMMENT ' ID number ',
PRIMARY KEY (`studentno`),
KEY `FK_gradeid` (`gradeid`),
CONSTRAINT `FK_gradeid` FOREIGN KEY (`gradeid`) REFERENCES `grade`
(`gradeid`)
) ENGINE=INNODB DEFAULT CHARSET=utf8


--  How to add statements using statements ?
--  grammar  : INSERT INTO  Table name [( Field 1, Field 2, Field 3,...)] VALUES(' value 1',' value 2',' value 3')
INSERT INTO grade(gradename) VALUES (' Freshman ');

--  The primary key increases automatically , Can we omit that ?
INSERT INTO grade VALUES (' Sophomore ');

--  Inquire about :INSERT INTO grade VALUE (' Sophomore ') Error code ： 1136
Column count doesn`t match value count at row 1

--  Conclusion :' Field 1, Field 2...' This part can be omitted  ,  But the added value must be consistent with the table structure , Data columns , The order corresponds to , And the quantity is one 
 Cause .

--  Insert multiple data at a time 
INSERT INTO grade(gradename) VALUES (' Junior year '),(' Senior ');

update command

grammar ：
UPDATE Table name SET column_name=value [,column_name2=value2,…] [WHERE condition];

Be careful :

• column_name For the data column to change
• value Is the modified data , It can be a variable , Specifically refers to , Expression or nested SELECT result
• condition Filter by , Modify all column data of the table if not specified

Operator :

--  Modify grade information 
UPDATE grade SET gradename = ' high school ' WHERE gradeid = 1;

DELETE command

DELETE FROM Table name [WHERE condition];

Be careful ：condition Filter by , Delete all column data of the table if not specified

-- Delete the last data DELETE FROM grade WHERE gradeid = 5

TRUNCATE command

effect ： Used to completely empty table data , But the table structure , Indexes , Constraints, etc., remain unchanged ;
grammar ：
TRUNCATE [TABLE] table_name;

Be careful ： The difference in DELETE command
identical : Can delete data , Do not delete table structure , but TRUNCATE Faster, different :

• Use TRUNCATE TABLE To reset AUTO_INCREMENT Counter
• Use TRUNCATE TABLE No impact on transactions （ It will be said after the transaction ）

test :

--  Create a test table 
CREATE TABLE `test` (
`id` INT(4) NOT NULL AUTO_INCREMENT,
`coll` VARCHAR(20) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8

--  Insert several test data 
INSERT INTO test(coll) VALUES('row1'),('row2'),('row3');

--  Delete table data ( No where Conditions of the delete)
DELETE FROM test;
--  Conclusion : If not specified Where Delete all column data of the table , The current value of auto increment is still based on the original value , Will log .

--  Delete table data (truncate)
TRUNCATE TABLE test;
--  Conclusion :truncate Delete data , The auto increment current value will return to the original value and start again ; No logging .

--  Also use DELETE Empty database table data of different engines . After restarting the database service 
-- InnoDB :  Auto increment column starts again from the initial value  ( Because it's stored in memory , If you cut off the power, you will lose )
-- MyISAM :  The auto increment column still starts from the previous auto increment data  ( In the file , Will not be lost )