MySQL is a very popular open source relational database management system that supports complete DDL (data definition language) operations. DDL is a language used to define and manage various data objects in the database, including data tables, views, indexes, etc. It is very important for database administrators and developers to be proficient in DDL operation technology of data tables in MySQL.
This article will introduce in detail the technology and methods of DDL operation of data tables in MySQL, and provide practical operation examples.
1. Create a data table
Creating a data table is one of the most basic operations in DDL. In MySQL, you can use the CREATE TABLE statement to create a data table. The basic syntax of this statement is as follows:
CREATE TABLE table_name ( column1 datatype constraints, column2 datatype constraints, ..... columnN datatype constraints, PRIMARY KEY (one or more columns) );
Among them, table_name is the name of the new data table, column1 to columnN are the columns of the table that needs to be created. Each column consists of a column name, data type, and constraints. Only primary key columns need to be constrained using the PRIMARY KEY keyword. In actual operation, table columns can be added and deleted according to needs, and constraints can also be set according to actual conditions.
The following is a practical example of creating a data table:
CREATE TABLE employee ( id INT(11) NOT NULL AUTO_INCREMENT, name VARCHAR(50) NOT NULL, age INT(11) NOT NULL, gender CHAR(1) NOT NULL, address VARCHAR(200) DEFAULT NULL, PRIMARY KEY (id) );
In the above example, a data table named employee is created, including id, name, age, gender and address. column and use the id column as the primary key. The id column has the AUTO_INCREMENT attribute set, which means that the id value of each row will be automatically incremented. The name, age, and gender columns all have NOT NULL constraints set, which means that these columns cannot have null values. The address column does not have a NOT NULL constraint set, which means it can be null.
2. Modify the data table
In MySQL, you can use the ALTER TABLE statement to modify the data table. Common modification operations include adding columns, deleting columns, modifying column definitions, and adding constraints.
1. Add columns
In order to add a new column to an existing table, you can use the ALTER TABLE statement and the ADD COLUMN clause. The following is an example of an operation to add a column:
ALTER TABLE employee ADD COLUMN position VARCHAR(50) NOT NULL;
This operation will add a new column named position at the end of the employee table. This column contains a VARCHAR(50) type of data and a NOT NULL constraint.
2. Delete a column
In order to delete an existing column, you can use the ALTER TABLE statement and the DROP COLUMN clause. The following is an example of an operation to delete a column:
ALTER TABLE employee DROP COLUMN address;
This operation will delete the column named address from the employee table.
3. Modify column definition
In order to modify the definition of an existing column, you can use the ALTER TABLE statement and MODIFY COLUMN clause. The following is an example of an operation to modify a column definition:
ALTER TABLE employee MODIFY COLUMN age INT(5) NOT NULL;
This operation will modify the data type of the age column of the employee table to INT (5), and add a NOT NULL constraint.
4. Add constraints
In order to add a constraint to an existing table and column, you can use the ALTER TABLE statement and the ADD CONSTRAINT clause. The following is an example of an operation to add constraints:
ALTER TABLE employee ADD CONSTRAINT chk_age CHECK (age>=18);
This operation will add a CHECK constraint to the age column of the employee table. This constraint requires that the value of the age column must be greater than or equal to 18.
3. Delete data table
In MySQL, you can use the DROP TABLE statement to delete an existing data table. You must be careful when using the DROP TABLE statement because all data associated with the table will be permanently deleted, including the table itself and all data rows, indexes, and constraints.
The following is an example of an operation to delete a data table:
DROP TABLE employee;
This operation will delete the data table named employee.
Conclusion
This article introduces the technology and methods of DDL operation of data tables in MySQL, including creating data tables, modifying data tables and deleting data tables. In actual operations, database administrators and developers should understand the basic concepts and syntax of DDL and flexibly use various DDL operations as needed. Proficient application of DDL can not only improve the management efficiency of MySQL database, but also effectively ensure the integrity and security of data.
The above is the detailed content of Data table DDL operation technology in MySQL. For more information, please follow other related articles on the PHP Chinese website!
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