SQL Syntax

The syntax of SQL is governed by the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO).

SQL Statements

SQL statements are very simple and straightforward like plain English but with specific syntax.

An SQL statement is composed of a sequence of keywords, identifiers, etc. terminated by a semicolon (;). Here is an example of a valid SQL statement.

SELECT emp_name, hire_date, salary FROM employees WHERE salary > 5000;

For better readability you can also write the same statement, as follow:

SELECT emp_name, hire_date, salary 
FROM employees 
WHERE salary > 5000;

Use semicolon at the end of an SQL statement — it terminates the statement or submits the statement to the database server. Some database management system has, however, no such requirement, but it is considered as a best practice to use it.

We'll discuss each part of these statements in detail in upcoming chapters.

Case Sensitivity in SQL

Consider another SQL statement that retrieves the records from employees table:

SELECT emp_name, hire_date, salary FROM employees;

The same statement can also be written, as follow:

select emp_name, hire_date, salary from employees;

SQL keywords are case-insensitive that means SELECT is same as select. But, the database and table names may case-sensitive depending on the operating system. In general, Unix or Linux platforms are case-sensitive, whereas Windows platforms aren't.

SQL Comments

A comment is simply a text that is ignored by the database engine. Comments can be used to provide a quick hint about the SQL statement.

SQL support single-line as well as multi-line comments. To write a single-line comment start the line with two consecutive hyphens (--). For example:

-- Select all the employees
SELECT * FROM employees;

However to write multi-line comments, start the comment with a slash followed by an asterisk (/*) and end the comment with an asterisk followed by a slash (*/), like this:

/* Select all the employees whose 
salary is greater than 5000 */
SELECT * FROM employees
WHERE salary > 5000;