SQL learning (1) - table related operations

Link to the original text ：https://github.com/datawhalechina/wonderful-sql
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1. Database creation and deletion

--  Create Syntax 
CREATE DATABASE <  Database name  > ;
--  Homework example 
create database task01;

--  Delete Syntax 
Drop DATABASE <  Database name  > ;

2. Create and delete tables

2.1. data type

INTEGER type
Used to specify the data type of the column where integers are stored （ Digital ）, Can't store decimals .

CHAR type
Used to store fixed length strings , When the length of the string stored in the column does not reach the maximum length , Use half space to complement , Because it wastes storage space , So I don't use .

VARCHAR type
Used to store variable length strings , Fixed length string will be supplemented with half width spaces when the number of characters does not reach the maximum length , But variable length strings are different , Even if the number of characters does not reach the maximum length , I don't use half space to make up .

DATE type
Used to specify the storage date （ Specific date ） The data type of the column of （ Date type ）.

2.2. Setting of constraints

Constraints are in addition to data types , The function of restricting or appending conditions to the data stored in a column .

NOT NULL Yes no empty constraint , That is, data must be entered in the column .

PRIMARY KEY It's a primary key constraint , Represents that the column is unique , You can get the data of a specific row through this column .

2.3. Create new table

--  grammar 
CREATE TABLE <  Table name  >
( <  Name  1> <  data type  > <  Constraints required for this column  > ,
  <  Name  2> <  data type  > <  Constraints required for this column  > ,
  <  Name  3> <  data type  > <  Constraints required for this column  > ,
  <  Name  4> <  data type  > <  Constraints required for this column  > ,
  .
  <  Constraints for this table  1> , <  Constraints for this table  2> ,……);
  
--  Homework example 
CREATE TABLE Addressbook
(regist_no INT  NOT NULL,
 name  VARCHAR(128) NOT NULL,
 address VARCHAR(256) NOT NULL,
 tel_no  VARCHAR(20) ,
 mail_address VARCHAR(20) ,
 PRIMARY KEY (regist_no));

ALTER TABLE Addressbook ADD COLUMN postal_code CHAR(8);

2.4. Delete table

Here's the thing to watch out for , Deleted tables cannot be recovered , It can only be re inserted , Please be very careful when deleting .
Delete table Syntax ：

DROP TABLE <  Table name  > ;

DROP TABLE Addressbook ;

--  Clear table contents 
-- advantage ： comparison drop / delete,truncate When used to clear data , The fastest .
TRUNCATE TABLE TABLE_NAME;

2.5. Column processing in the table

ALTER TABLE Statement and DROP TABLE The sentence is the same , Cannot recover after execution . Mistakenly added columns can be through ALTER TABLE Statement delete , Or delete all the tables and create them again .

--  Insert new column 
ALTER TABLE <  Table name  > ADD COLUMN <  Definition of columns  >;
ALTER TABLE Addressbook ADD COLUMN postal_code VARCHAR(8);
--  Delete column 
ALTER TABLE <  Table name  > DROP COLUMN <  Name  >;
--  Delete a specific row in the table （ grammar ）
--  Be sure to add  WHERE  Conditions , Otherwise, all data will be deleted 
DELETE FROM product WHERE COLUMN_NAME='XXX';

3. Data processing in the table

3.1. Insert one line

After the table is created , You can insert data into the table

--  Basic grammar 
INSERT INTO < Table name > ( Column 1,  Column 2,  Column 3, ……) VALUES ( value 1,  value 2,  value 3, ……);  
--  Examples 
--  Contains a list of columns 
INSERT INTO Addressbook (regist_no,name,address,tel_no,mail_address,postal_code) 
VALUES (001,'zxx','DHU',139,'[email protected]',201620);  
--  Omit the list 
INSERT INTO Addressbook VALUES (001,'zxx','DHU',139,'[email protected]',201620);  

3.2. Multi line insertion

INSERT INTO Addressbook VALUES (001,'zxx','DHU',139,'[email protected]',201620),
(002,'zxy','DHU',139,'[email protected]',201620),
(003,'wzy','DHU',139,'[email protected]',201620);  

3.3. Copy the data

have access to INSERT … SELECT Statement to copy data from other tables .

--  Copy the data in the address table to the address replication table 
INSERT INTO Addressbookcopy (regist_no,name,address,tel_no,mail_address,postal_code)
SELECT regist_no,name,address,tel_no,mail_address,postal_code
  FROM Addressbook;  

3.4. Update and delete data

3.4.1. Single column update

--  grammar 
UPDATE < Table name >
   SET < Name > = < expression > [, < Name 2>=< expression 2>...]  
 WHERE < Conditions >  --  Optional , It's very important 
 ORDER BY  Clause   -- Optional 
 LIMIT  Clause ; -- Optional 

Use update Pay attention to adding where Conditions , Otherwise, all rows will be modified according to the statement

--  Modify all registration numbers 
UPDATE Addressbook
  SET regist_no = 001;  
--  Only modify the number of some people 
UPDATE Addressbook
  SET regist_no = regist_no * 10
WHERE name = 'zxx';  

3.4.2. Multi column update

UPDATE Of the statement SET Clause supports multiple columns as update objects at the same time .

--  Basic writing , One UPDATE Statement updates only one column 
UPDATE Addressbook
   SET regist_no = regist_no * 10
 WHERE name = 'wzy';
UPDATE Addressbook
   SET regist_no = regist_no + 10
 WHERE name = 'zxy';  

This writing can get the correct result , But the code is cumbersome . You can simplify the code by merging .

--  The combined writing 
UPDATE Addressbook
   SET regist_no = regist_no * 10,
       postal_code = postal_code +10
 WHERE name = 'zxx';  

Update error ：Error Code:1175.You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column.
mysql This error occurs when the delete update statement is executed , Because in mysql stay safe-updates In the pattern , If you where The following condition is not a primary key id, Then there will be such a mistake .

There are two solutions

--  Execute this command to change mysql database schema .
SET SQL_SAFE_UPDATES = 0;

--  stay where Follow the primary key in the judgment condition id
UPDATE Addressbook
  SET regist_no = regist_no * 10
WHERE name = 'gyt'and regist_no>0;

4. Exercises Task01

-- 1.1
create database task01;
use task01;
CREATE TABLE Addressbook
(regist_no INT  NOT NULL,
 name  VARCHAR(128) NOT NULL,
 address VARCHAR(256) NOT NULL,
 tel_no  Char(10) ,
 mail_address char(10) ,
 PRIMARY KEY (regist_no));
 
 -- 1.2
ALTER TABLE Addressbook ADD COLUMN postal_code CHAR(8);
select * from Addressbook;

-- 1.3
--  Please add the following  SQL  Statement to delete  Addressbook  surface .
drop  table  Addressbook;

-- 1.4  Judgment questions 
--  Can I write  SQL  Statement to restore deleted  Addressbook  surface ？
--  Deleted tables cannot be recovered , It can only be re inserted , Please be very careful when deleting .