SQL MDN Docs WHERE

WHERE clause in SQL

The WHERE clause in SQL is used for filtering records. It allows you to specify conditions that each row of the table must meet to make it to the result set (query result). This makes the WHERE clause essential for making any data analysis or data operation.

The WHERE clause can be used with any SQL statement: SELECT, UPDATE, DELETE or INSERT.

Syntax

The basic syntax of a WHERE clause looks like this:

SELECT column1, column2
FROM table
WHERE condition

In such a query, SQL engine goes over every row inside the table and checks whether it satisfies the condition (when you hear “condition” it basically means a statement that returns either TRUE or FALSE).

The condition in the WHERE clause supports various operators such as =, !=, <, >, LIKE, IN, etc., to specify the criteria the data must meet.

We can also combine multiple filters using the AND or OR operators. Let’s look at some examples.

Filtering data with WHERE

Using comparison operators

You can use comparison operators to filter data based on numeric comparison:

SELECT *
FROM transactions
WHERE
  amount_usd > 100

String pattern matching with LIKE

The LIKE operator is used for pattern matching in string columns. You can use % as a wildcard character:

SELECT *
FROM users
WHERE
  email NOT LIKE '%@gmail.com'

Filtering with multiple conditions

You can combine multiple conditions using AND and OR operators:

SELECT *
FROM users
WHERE
  country = 'us'
  AND age >= 21

Understanding and utilizing the WHERE clause effectively is a cornerstone of bulding an SQL habit – a state where we can type queries faster than English and answer any question with data.

Mastering the WHERE clause is essential for querying data, precise data analysis and manipulation.