What is SQL?

SQL is a standard Database language which is used to create, maintain and retrieve the data from relational databases like MySQL, Oracle, SQL Server, PostGre, etc

• SQL stands for Structured Query Language
• It lets you access and manipulate data from databases

RDBMS

RDBMS stands for Relational Database Management System.

It is the basis for SQL, and for all modern database systems such as MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.

The data in RDBMS is stored in database objects called tables. A table is a collection of related entries and it consists of columns and rows.

Database Tables

A database most often contains one or more tables. Each table is identified by a name.

SQL Statements

Most of the actions you need to perform on a database are done with SQL statements.

Semicolon after SQL Statements?

Some database systems require a semicolon at the end of each SQL statement.

Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.

Keep in Mind That…

• SQL keywords are NOT case sensitive: select is the same as SELECT

Some of The Most Important SQL Commands

• SELECT – extracts data from a database
• UPDATE – updates data in a database
• DELETE – deletes_data from a database
• INSERT INTO – inserts new_data into a database
• CREATE DATABASE – creates a new database
• ALTER DATABASE – modifies a database
• CREATE TABLE – creates a new table
• ALTER TABLE – modifies a table
• DROP TABLE – deletes a table
• CREATE INDEX – creates an index (search key)
• DROP INDEX – deletes an index

The SQL SELECT Statement

The SELECT statement is used to select data from a database.

The data returned is stored in a result table called the result-set.

SELECT Syntax

SELECT column1, column2, …
FROM table_name;

Here, column1, column2, … are the field names of the table you want to select data from. To select all the fields available in the table, use the following syntax:

SELECT * FROM table_name;

The SQL SELECT DISTINCT Statement

The SELECT DISTINCT statement is used to return only distinct (different) values.

Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values.

SELECT DISTINCT Syntax

SELECT DISTINCT column1, column2, …
FROM table_name;

The following SQL statement lists the number of different (distinct) customer countries:

Example

SELECT COUNT(DISTINCT Country) FROM Customers;

Note: The example above will not work in Firefox! Because COUNT(DISTINCT column_name) is not supported in Microsoft Access databases. Firefox is using Microsoft Access in our examples.

Here is the workaround for MS Access:

Example

SELECT Count(*) AS DistinctCountries
FROM (SELECT DISTINCT Country FROM Customers);

The SQL WHERE Clause

The WHERE clause is used to filter records.

It is used to extract only those records that fulfill a specified condition.

WHERE Syntax

SELECT column1, column2, …
FROM table_name
WHERE condition;

Note: The WHERE clause is not only used in SELECT statements, it is also used in UPDATE, DELETE, etc.!

Operators in The WHERE Clause

The following operators can be used in the WHERE clause:

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