100 important knowledge points that SQL must master: creating and manipulating tables

17.1 Create table
SQL Not only for table data manipulation , It is also used to perform all operations on databases and tables , package
Including the creation and processing of the table itself .
There are generally two ways to create tables ：
 Most of the DBMS Both have interactive tools for creating and managing database tables ;
 Tables can also be used directly SQL Statement manipulation .
Create a table with a program , have access to SQL Of CREATE TABLE sentence . It should be noted that , send
When you use interactive tools, you actually use SQL sentence . These statements are not written by the user , world
The face tool automatically generates and executes the corresponding SQL sentence （ The same is true when changing an existing table ）.
Be careful ： Grammatical differences
In different SQL In the implementation , CREATE TABLE The syntax of the statement may be different . Yes
Specific DBMS What syntax is supported , Please refer to the corresponding documentation .
This lesson will not cover all the options available when creating tables , That's beyond the scope of this lesson ,
I'll just give you some basic options . Detailed information description , Please refer to the specific DBMS file .
explain ： Various DBMS Specific examples of creating tables
About difference DBMS Of CREATE TABLE Specific examples of statements , Please refer to the appendix A in
The sample table creation script .
17.1.1 Table creation foundation
utilize CREATE TABLE Create table , The following information must be given ：
 Name of the new table , In keywords CREATE TABLE Then give ;
 Table column names and definitions , Separate with commas ;
 yes , we have DBMS It is also required to specify the location of the table .
Below SQL Statement to create the... Used in this book Products surface ：
Input ▼
CREATE TABLE Products
(
prod_id CHAR(10) NOT NULL,
vend_id CHAR(10) NOT NULL,
prod_name CHAR(254) NOT NULL,
prod_price DECIMAL(8,2) NOT NULL,
prod_desc VARCHAR(1000) NULL
);
analysis ▼
As you can see from the above example , Table name follows CREATE TABLE keyword . The actual table is fixed
The righteous （ All columns ） Enclosed in parentheses , The columns are separated by commas . This watch consists of 5 Line up
become . Each column is defined by column name （ It must be unique in the table ） Start , Data followed by column
type （ Explanation of data types , Please refer to the first 1 course . Besides , appendix C Lists common
Data type and compatibility ）. The entire statement ends with a semicolon after parentheses .
Mentioned earlier , Different DBMS Of CREATE TABLE The grammar of is different , This simple foot
Ben also explains this . This statement is used in most DBMS Effective in , But for the DB2,
Must be removed from the last column NULL . This is for different DBMS, To write different
Table reasons for creating scripts （ See Appendix A）.
Tips ： Statement formatting
Think back in SQL Space ignored in statement . Statement can be entered on a long line , also
It can be divided into many lines , They make no difference . such , You can use the best formula for yourself
Arrange the format of the statement . Ahead CREATE TABLE The statement is SQL Statement formatting
A good example of , The code is arranged on multiple lines , Column definitions are properly indented , more
Easy to read and edit . In what format SQL The statement does not specify , But I highly recommend
Use some sort of indentation format .
Tips ： Replace existing table
When creating a new table , The specified table name must not exist , Otherwise it will go wrong . Prevent accidental cover
Cover the existing watch ,SQL It is required to delete the table manually first （ See the following ）, however
Then rebuild it , Instead of simply overwriting it with a CREATE TABLE statement .
17.1.2 Use NULL value
The first 4 Class mentioned , NULL Value means no value or missing value . allow NULL Value columns are also allowed to insert
The value of this column is not given when entering a row . Don't allow NULL Columns with values do not accept rows without column values , in
In a word , When inserting or updating rows , The column must have a value .
Each table column is either NULL Column , Or NOT NULL Column , This state is created by the table
The definition of defines . Please see the following example ：
Input ▼
CREATE TABLE Orders
(
order_num INTEGER NOT NULL,
order_date DATETIME NOT NULL,
cust_id CHAR(10) NOT NULL
);
analysis ▼
This statement creates the Orders surface . Orders It contains three columns ： The order number 、 book
Single date and customer ID. All three columns need , Therefore, the definition of each column contains keywords NOT
NULL . This prevents the insertion of columns without values . If you insert a column without a value , Will return an error ,
And the insertion failed .
The next example will create a hybrid NULL and NOT NULL List of columns ：
Input ▼
CREATE TABLE Vendors
(
vend_id CHAR(10) NOT NULL,
vend_name CHAR(50) NOT NULL,
vend_address CHAR(50) ,
vend_city CHAR(50) ,
vend_state CHAR(5) ,
vend_zip CHAR(10) ,
vend_country CHAR(50)
);
analysis ▼
This statement creates the Vendors surface . supplier ID And the name of the supplier
Required , So it's designated as NOT NULL . All the other five columns allow NULL value , So don't specify NOT
NULL . NULL For default settings , If you don't specify NOT NULL , I think the designation is NULL .
Be careful ： Appoint NULL
Not specified NOT NULL when , Most of the DBMS It is thought that what is specified is NULL , But not all
Of DBMS It's all like this . some DBMS Require keyword to be specified NULL , If not specified, it will
error . For complete syntax information , Please refer to the specific DBMS file .
Tips ： Primary key and NULL value
The first 1 The class has introduced , A primary key is a column whose value uniquely identifies each row in a table . Only not allowed NULL
Value column can be used as primary key , allow NULL The column of the value cannot be used as a unique identity .
Be careful ： understand NULL
Don't put the NULL Values are confused with empty strings . NULL Value is no value , Not an empty string .
If specified '' （ Two single quotes , No characters in between ）, This is in NOT NULL Column is allowed
Xu's . An empty string is a valid value , It's not worthless . NULL Key for value NULL
Instead of an empty string specifying .
17.1.3 Specify default
SQL Allows you to specify default values , If no value is given when inserting a row ,DBMS The default will be automatically applied
Recognition . The default value is in CREATE TABLE Use the keyword... In the column definition of the statement DEFAULT Appoint .
Please see the following example ：
Input ▼
CREATE TABLE OrderItems
(
order_num INTEGER NOT NULL,
order_item INTEGER NOT NULL,
prod_id CHAR(10) NOT NULL,
quantity INTEGER NOT NULL DEFAULT 1,
item_price DECIMAL(8,2) NOT NULL
);
analysis ▼
This statement creates OrderItems surface , Contains the items that make up the order （ The order itself is stored in
Orders In the table ）. quantity List the quantity of each item in the order . In this case , this
A column description is added DEFAULT 1 , instructions DBMS, If no quantity is given, use quantity 1 .
Default values are often used for date or timestamp columns . for example , By specifying a function that references the system date
Or variable , Use the system date as the default date .MySQL User specified DEFAULT CURRENT_
DATE() ,Oracle User specified DEFAULT SYSDATE , and SQL Server User specified
DEFAULT GETDATE() . Unfortunately , This command to get the system date is in different
DBMS Most of them are different . surface 17-1 This command is listed in some DBMS Chinese grammar .
If there is no one listed here DBMS, Please refer to the corresponding documentation .
surface 17-1 Get the system date
DBMS function / Variable
DB2 CURRENT_DATE
MySQL CURRENT_DATE()
Oracle SYSDATE
PostgreSQL CURRENT_DATE
SQL Server GETDATE()
SQLite date('now')
Tips ： Use DEFAULT instead of NULL value
Many database developers like to use DEFAULT It's worth it, not NULL Column , For the use of
This is especially true for columns of calculation or data grouping .
17.2 Update table
Update table definition , have access to ALTER TABLE sentence . Although all DBMS All support
ALTER TABLE , But what they allow to be updated is very different . Here are the USES ALTER
TABLE Things to consider when .
 Ideally , Do not update a table when it contains data . Should be in the table design
During the process, the possible future needs shall be fully considered , Avoid major changes to the structure of the table in the future .
 be-all DBMS Are allowed to add columns to existing tables , However, for the data type of the added column
（ as well as NULL and DEFAULT Use ） There are limits .
 many DBMS Deleting or changing columns in a table is not allowed .
 Most of the DBMS Allow renaming columns in a table .
 many DBMS Restrict changes to columns already filled with data , For columns not filled with data
There is almost no limit .
It can be seen that , Making changes to existing tables is complex and inconsistent . What changes can be made to the table structure
Change , Please refer to the specific DBMS file .
Use ALTER TABLE Change table structure , The following information must be given ：
 stay ALTER TABLE Then give the table name to be changed （ The table must exist , Otherwise, it will be wrong ）;
 List what changes to make .
Because adding columns to an existing table may be all DBMS The only operation supported by both , So we take
An example of this ：
Input ▼
ALTER TABLE Vendors
ADD vend_phone CHAR(20);
analysis ▼
This statement gives Vendors Add a table named vend_phone The column of , Its data type
by CHAR .
Change or delete columns 、 Add constraints or keys , These operations use a similar syntax .
Be careful , The following example is not for all DBMS Are effective ：
Input ▼
ALTER TABLE Vendors
DROP COLUMN vend_phone;
Complex table structure changes generally require manual deletion process , It involves the following steps ：
(1) Create a new table with a new column layout ;
(2) Use INSERT SELECT sentence （ A detailed introduction to this statement , Please refer to the first 15
course ） Copy data from old table to new table . If necessary , You can use conversion functions and calculations
Field ;
(3) Verify the new table containing the required data ;
(4) Rename old table （ If determined , You can delete it ）;
(5) Rename the new table with the original name of the old table ;
(6) According to need , Recreate trigger 、 stored procedure 、 Indexes and foreign keys .
explain ： ALTER TABLE and SQLite
SQLite To use ALTER TABLE There are restrictions on what to do . The most important limitation
yes , It doesn't support the use of ALTER TABLE Define primary and foreign keys , These must be created at the beginning
When creating a table, specify .
Be careful ： Careful use ALTER TABLE
Use ALTER TABLE Be extremely careful , A full backup should be made before making changes （ surface
Backup of structure and data ）. Changes to database tables cannot be undone , If you add unnecessary
Column , Maybe you can't delete them . Similarly , If you delete a column that should not be deleted , Probably
All data in this column will be lost .
17.3 Delete table
Delete table （ Delete the entire table instead of its contents ） It's simple , Use DROP TABLE sentence
that will do ：
Input ▼
DROP TABLE CustCopy;
analysis ▼
This statement deletes CustCopy surface （ The first 15 Created in class ）. There is no confirmation step for deleting the table ,
It can't be revoked , Executing this statement will permanently delete the table .
Tips ： Use relationship rules to prevent accidental deletion
many DBMS Allow enforcement of relevant rules , Prevent deleting tables associated with other tables . stay
When implementing these rules , If a table is published DROP TABLE sentence , And the table is
Part of a relationship , be DBMS Will prevent this statement from executing , Until the relationship is deleted
Until . If allowed , These options should be enabled , It prevents accidental deletion of useful tables .
17.4 rename table
Every DBMS Support for table renaming is different . For this operation , There is no strict
standard .DB2、MariaDB、MySQL、Oracle and PostgreSQL The user to use RENAME
sentence ,SQL Server The user to use sp_rename stored procedure ,SQLite The user to use ALTER
TABLE sentence .
The basic syntax of all renaming operations requires specifying the old table name and the new table name . however , There is
DBMS Realize the difference . About specific grammar , Please refer to the corresponding DBMS file .
author ： Guge academic

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