What is the purpose of the WHERE clause in an SQL query?

The WHERE clause in an SQL query is used to filter records that fulfil a specified condition.

In more detail, the WHERE clause is a fundamental part of SQL (Structured Query Language), which is used to manipulate and retrieve data stored in relational databases. The WHERE clause is used in conjunction with SQL statements like SELECT, UPDATE, DELETE, etc., to filter the results of the statement. It allows you to specify a condition that the records must meet to be included in the result set.

For example, if you have a database table of students and you want to retrieve only those students who are studying Computer Science, you would use a WHERE clause to specify this condition. The SQL query might look something like this: SELECT * FROM students WHERE course = 'Computer Science'. This query would return all records from the 'students' table where the 'course' field is equal to 'Computer Science'.

The WHERE clause can also be used with logical operators like AND, OR, and NOT to combine or negate conditions, providing a great deal of flexibility in defining the criteria for the data you want to retrieve. For instance, you could modify the above query to: SELECT * FROM students WHERE course = 'Computer Science' AND year = '2'. This would return only the records of second-year Computer Science students.

Furthermore, comparison operators such as = (equal), <> (not equal), > (greater than), < (less than), >= (greater than or equal), <= (less than or equal) can be used in the WHERE clause to further refine the conditions.

In summary, the WHERE clause is a powerful tool in SQL that allows you to filter data based on specific conditions, making it easier to find and manipulate the data you need. It is a fundamental part of most SQL queries and is essential for effective database management.