WHEREklausuli

The WHERE clause in SQL is a fundamental component used to filter records. It is employed in SELECT, UPDATE, and DELETE statements to specify which rows should be affected by the operation. The clause consists of a condition that evaluates to true or false for each row in the table. Only rows for which the condition evaluates to true are included in the result set or are modified.

Conditions within a WHERE clause are typically formed using comparison operators such as equals (=), not equals (!= or <>), greater than (>), less than (<), greater than or equal to (>=), and less than or equal to (<=). Logical operators like AND, OR, and NOT can be used to combine multiple conditions, creating more complex filtering logic. For instance, a WHERE clause might specify that a record must meet criteria A AND criteria B, or it could specify that it must meet criteria A OR criteria B.

Other useful operators include LIKE for pattern matching in strings, IN to check if a value exists within a list of values, BETWEEN to check if a value falls within a range, and IS NULL to check for the absence of a value. The NULL keyword is a special marker that represents a missing or unknown value.

Effectively utilizing the WHERE clause is crucial for efficient data retrieval and manipulation in relational databases. It allows users to isolate specific data points based on defined criteria, making database operations more targeted and manageable. Without the WHERE clause, SQL statements would operate on all rows in a table, which is often undesirable and can lead to performance issues or incorrect results.