DDL (Data Definition Language) Commands in MySQL

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DDL statements or commands are used to define and modify the database structure of your tables or schema. When you execute a DDL statement, it takes effect immediately.

Some commands of DDL are:

  • CREATE – to create table (objects) in the database
  • ALTER – alters the structure of the database
  • DROP – delete table from the database
  • TRUNCATE – remove all records from a table, including all spaces allocated for the records are removed
  • COMMENT – add comments to the data dictionary
  • RENAME – rename a table

CREATE :

The create table statement (query) to create a table is given below:

 CREATE TABLE <table name> (
 <attribute name 1> <data type 1>,
 ...
 <attribute name n> <data type n>);

Example:

CREATE TABLE STUDENT ( StudID NUMBER, Name VARCHAR);

The data types that you will use most frequently are character strings, which might be called VARCHAR or CHAR for variable or fixed length strings; numeric types such as NUMBER or NTEGER, which will usually specify a precision; and DATE or related types. Data types are differ according to the databases software whatever you are using to your system.

ALTER :

The alter table statement to make modifications to the table structure such as Key constraints, Column size, etc.

ALTER TABLE <table name> ADD CONSTRAINT <constraint name> PRIMARY KEY(<attribute list>);

Example:

ALTER TABLE STUDENT ADD CONSTRAINT NOT NULL PRIMARY KEY (StudID);

DROP :

The drop table statement (query) to delete a table is given below:

DROP TABLE <table name>;

Example:

DROP TABLE STUDENT;
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