Defining Database Schemas with SQL CREATE TABLE Statements
In database design, use the CREATE TABLE statement to define table structures and constraints to ensure data integrity. 1. Each table needs to specify a field, data type and primary key, such as user_id INT PRIMARY KEY; 2. Add NOT NULL, UNIQUE, DEFAULT and other constraints to improve data consistency, such as email VARCHAR(255) NOT NULL UNIQUE; 3. Use FOREIGN KEY to establish an association between tables, such as the orders table references the primary key of the users table through user_id.
In database design, a clear structure is the first step. Using SQL's CREATE TABLE statement, you can specify fields, types, and constraints for a table to create a solid data foundation.
Understand basic syntax structure
Each CREATE TABLE statement starts with a keyword, followed by the table name and column definitions in parentheses. Each column requires a name, data type, and optional constraints. For example:
- Column names are usually underlined in lowercase letters (such as
user_id) - Common data types include
INT,VARCHAR(n),DATE,BOOLEAN, etc. - The primary key is specified by
PRIMARY KEYto uniquely identify each row
A typical example is as follows:
CREATE TABLE users (
user_id INT PRIMARY KEY,
name VARCHAR(100),
email VARCHAR(255),
created_at DATE
); This statement creates a table called users , contains four fields, and set user_id as the primary key.
Add constraints to improve data integrity
In addition to primary keys, other constraints can be used to ensure the consistency and validity of data. Commonly used are:
-
NOT NULL: Make sure that a column cannot be NULL -
UNIQUE: Make sure that the values in the column are unique -
DEFAULT: Provides default values for columns -
FOREIGN KEY: Establish an association relationship with other tables
For example, if you want email field not to be empty and unique, you can write it like this:
CREATE TABLE users (
user_id INT PRIMARY KEY,
name VARCHAR(100),
email VARCHAR(255) NOT NULL UNIQUE,
created_at DATE DEFAULT CURRENT_DATE
); Here, the default value for created_at is set to the current date. If the field is not specified when inserting a new record, it will be automatically filled.
Use foreign keys to create connections between tables
In a multi-table system, foreign keys are used to maintain reference integrity between different tables. Assume there is also an orders table, which should be associated with the users table, which can be defined like this:
CREATE TABLE orders (
order_id INT PRIMARY KEY,
user_id INT,
amount DECIMAL(10,2),
FOREIGN KEY (user_id) REFERENCES users(user_id)
); This means that user_id in orders table must exist in user_id of the users table to prevent invalid user order records.
It should be noted that different databases have slightly different support for foreign keys. Some lightweight databases (such as SQLite) may not be enforced although they support syntax.
Basically that's it. After mastering these basic elements, you can use CREATE TABLE to build a database model with reasonable structure and clear relationships.
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