Question

Which SQL clause is used to filter rows based on a condition ?

• A. ORDER BY
• B. WHERE
• C. GROUP BY
• D. HAVING

Answer 1

Answer:

B. WHERE

The SQL WHERE Clause

• The WHERE clause is fundamental in SQL for data retrieval, enabling the filtering of records that meet specific criteria defined by the user.
• It is used in conjunction with SELECT, UPDATE, and DELETE statements to restrict operations to a subset of the dataset.
• Syntax: SELECT column1, column2 FROM table_name WHERE condition;.

Key Operators used with WHERE:

• Comparison Operators: =, <>, !=, <, <=, >, >=.
• Logical Operators: AND, OR, NOT, which allow for combining multiple conditions to narrow down results.
• Pattern Matching: The LIKE operator is used for pattern matching within string columns (e.g., using % or _ wildcards).
• Range Operators: BETWEEN is used to filter values within a specific range, including the boundary values.
• Set Membership: IN is used to specify multiple possible values for a column.
• Null Check: IS NULL or IS NOT NULL is required to identify missing data, as standard comparison operators do not function correctly with NULL values.

Execution Order

• In the logical processing of a SQL query, the WHERE clause is evaluated before the GROUP BY clause.
• Rows that do not satisfy the condition in the WHERE clause are filtered out before any aggregation (such as SUM, COUNT, or AVG) takes place.
• To filter data after aggregation, the HAVING clause must be used instead.