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A comma-separated list of the column names follows the SELECT clause. The FROM clause is followed by one or more table names, and the WHERE clause is a condition to filter records.

SELECT column1, column2, column3
FROM table_name;
WHERE condition

A simple SELECT command selects all records from the table, Customers.

SELECT * FROM Customers

The following SELECT command selects three columns from the Customers table in London city.

SELECT ContactName, ContactTitle, City FROM Customers WHERE City='London'

SQL SELECT query with WHERE clause can have many operators to filter records based on conditions. We can also select records in sorted orders from the database. We can also search records between a range.

SQL as Data Definition Language

SQL DDL (Data Definition Language) commands create and manipulate database objects, including the database itself, tables, and other objects. The most commonly used DDL commands are CREATE, ALTER, and DROP.

Here are examples of CREATE, ALTER, and DROP commands.

CREATE DATABASE command creates a new database.

CREATE DATABASE databasename;  

CREATE TABLE creates a new database table.  

CREATE TABLE tablename (  
column1 datatype,  
column2 datatype,  
column3 datatype,  

);  

​The following CREATE TABLE command creates a new table with three columns.

CREATE TABLE Authors (  
AuthorID int,  
Name varchar(255),  
Book varchar(255),  
Publisher varchar(255)
);  

ALTER TABLE command alters a database table schema. For example, if you want to change a column's data type or add a new column.

ALTER TABLE tablename

ADD columnname datatype;

The following command alters the Authors table by adding a new column.

ALTER TABLE Authors  
ADD Email varchar(255);  

DROP DATABASE command deletes a database.

DROP DATABASE databasename;

DROP TABLE deletes a database table.

DROP TABLE tablename;

The following command deletes the Authors table.

​DROP TABLE Authors;

SQL as Data Manipulation Language

SQL provides DML (Data Manipulation Language) commands to insert, update, and delete data. Common DML commands are INSERT, UPDATE, and DELETE.

Here are some examples of INSERT INTO, UPDATE, and DELETE commands.

INSERT INTO command inserts new records to a database table.

INSERT INTO TableName (Column1, Column2, Column3) VALUES (value1, value2, value3);

The following SQL command inserts one record.

INSERT INTO Authors (AuthorID, Name, Book, Publisher)  VALUES (1001, 'Mahesh Chand', 'Programming ADO.NET', 'APress');  

UPDATE command updates existing records. The following UPDATE SQL command updates an existing record where AuthorID = 1001.

UPDATE Authors  
SET AuthorID = 100, Book ='C# 8'  
WHERE AuthorID = 1001  

SQL Update queries can be complex. Here is a detailed tutorial on SQL UPDATE Tutorial.

The DELETE command deletes existing records. The following DELETE command deletes existing records where AuthorID is 100.

DELETE FROM Authors WHERE AuthorID = 100

SQL as Data Control Language

SQL DCL (Data Control Language) commands control access to a database, including privileges on database objects. Common DCL commands are GRANT and REVOKE.

SQL Performance

SQL is the language to access RDBMS; if your SQL statements are not written well, it will affect the performance of your query and hence the application. It is highly recommended to learn how to optimize SQL statements.

Download free SQL EBooks.

Here are two free eBooks downloads (PDF) on SQL queries for beginners.

Joins in SQL

SQL Joins are used when you need to get data from multiple tables. There are four types of joins, inner, outer, cross, and self. If not programmed correctly, joins could affect an application's performance. Here is a detailed article on Joins in SQL Server. Everything You Need to Know about SQL Joins.

Summary

This article teaches SQL, its types, and how SQL commands are created and used in relational database management systems.