sql where

What is SQL WHERE?

SQL (Structured Query Language) WHERE clause is used to filter the records selection from a database table based on specified conditions. The WHERE clause will only return the records that meet the conditions. It can be used with SELECT, UPDATE and DELETE statements.

How does SQL WHERE work?

To use SQL WHERE, you need to identify the table and fields you want to search as well as the criteria to filter the records. For example, if you have a table of customers with fields like CustomerID, FirstName, LastName, and Email, and you want to find all customers with the last name SMITH, you would use the following statement:

SELECT * FROM customers WHERE LastName = 'Smith';

This statement would return all records with the last name Smith.

Types of SQL WHERE conditions

There are different types of conditions that can be used with SQL WHERE, including:

• Basic Operators: These include =, , >, = and <= which are used to compare two values.
• Logical Operators: These include AND, OR and NOT which are used to combine two or more conditions.
• Wildcards: These include % (percent) and _ (underscore) which are used to match any character or any single character respectively.

Examples of SQL WHERE statements

Here are some examples of SQL WHERE statements:

• SELECT * FROM customers WHERE LastName = 'Smith';
• SELECT * FROM products WHERE Price > 10;
• SELECT * FROM payments WHERE Date BETWEEN '2020-01-01' AND '2020-12-31';
• SELECT * FROM orders WHERE CustomerID IN (SELECT CustomerID FROM customers WHERE Country = 'USA');

Best practices for using SQL WHERE

To use SQL WHERE effectively, you should:

• Avoid using WHERE clauses with the LIKE operator on large datasets.
• Use WHERE clauses with column names instead of table aliases.
• Use parentheses to group and clarify conditions with multiple AND and OR operators.
• Avoid using negative conditions like NOT, as they can cause performance issues.
• Optimize queries by using indexes on columns that are searched or filtered.