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How to use the where clause with the select statement in SQL

Onyejiaku Theophilus Chidalu

Overview

The WHERE clause in SQL is used to extract records that meet a specified condition from a given table. The WHERE clause is commonly used with the comparison operators. They include:

  1. = (Equal to operator)
  2. != (Not Equal to operator)
  3. > ( Greater than operator)
  4. < ( Less than operator)
  5. >= (Greater than or Equal to operator)
  6. <= (Less than or Equal to operator)

Be that as it may, it is worth noting that:

  • If the WHERE clause has the primary key with the = operator, then only one record can be affected, if found.
  • Similarly, like the case above, if the WHERE clause has the non-primary key with the = operator, then multiple records can be affected, if found.
  • Irrespective of the key type ( either primary or non-primary key) with the inequality operators < or >, then multiple records are affected, if found.

Syntax

The syntax of the WHERE clause alongside the SELECT statement is given below:

SELECT column1, column2,...
FROM name_of_table
WHERE condition;

Database schema

We’ll work with the following database schema:

Customers

CustomerID

CustomerName

Country

1

Theo David

U.S.A

2

Peter Mark

United Kingdom

3

Antonio Clark

Mexico

4

Benjamin Monday

U.S.A

Example

In the code below, we’ll use the WHERE clause alongside the SELECT statement to retrieve data from the Customers table.

-- using the WHERE clause alongside the SELECCT statement
SELECT * FROM Customers
WHERE CustomerID=1;
Implementing the WHERE clause alongside the SELECT statement

Explanation

  • We declare the Customers table and populate it with the values according to the defined database schema on our platform.
  • In the statement shown above, we apply the WHERE clause alongside the SELECT statement with a condition of where the CustomerID is 1.

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Onyejiaku Theophilus Chidalu
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