SQL Server Tutorial - Part 16

Data Definition Language (DDL) in SQL

• DDL consists of SQL commands used to define the database schema.
• It is used to create and modify the structure of database objects.
• Key DDL statements: CREATE, ALTER, DROP, TRUNCATE, COMMENT.

Create Statement

• Used to create databases or objects (tables, indexes, functions, views, stored procedures, triggers).
• Syntax: CREATE <object type> <object name> (column definitions).
• Example:

  CREATE DATABASE testdb;
  CREATE TABLE test_table (id INT);
  

Alter Statement

• Used to add, modify, drop, or rename columns or tables.
• Syntax: ALTER TABLE <table name> <alteration>.
• Example:

  ALTER TABLE test_table ADD address NVARCHAR(500);
  

Drop Statement

• Used to remove databases, tables, indexes, triggers, constraints.
• Syntax: DROP <object type> <object name>.
• Example:

  DROP TABLE test_table;
  

Truncate Statement

• Used to remove all records from a table, freeing space allocated.
• Syntax: TRUNCATE TABLE <table name>.
• Example:

  TRUNCATE TABLE test_table;
  

Comment Statement

• Used to add comments to SQL queries.
• Types: Single-line, Multi-line, Inline.

Single-Line Comments

• Single line comments start with -- and do not execute.
• Syntax: -- Comment text
• Example:

  -- This is a single-line comment.
  SELECT * FROM test_table;
  

Multi-Line Comments

• Multi-line comments start with /* and end with */.
• Syntax: /* Comment text */
• Example:

  /* This is a 
  multi-line comment. */
  SELECT * FROM test_table;
  

Inline Comments

• Inline comments are an extension of multi-line comments and appear within statements.
• Syntax: /* Comment text */ within a statement
• Example:

  SELECT /* inline comment */ * FROM test_table;
  

Summary

• DDL in SQL includes commands to create and modify database schemas and objects.
• Key DDL statements are CREATE, ALTER, DROP, TRUNCATE, and COMMENT.
• Each statement plays a crucial role in managing the structure and data within SQL databases.

Next session: Discussing Data Query Language (DQL) and the SELECT statement.