SQL WHERE Clause

Understanding the SQL WHERE Clause

The WHERE clause is used in SQL to filter the results returned by a query. It allows you to specify conditions that rows must satisfy to be included in the result set.

Basic Syntax

Common SQL WHERE Clause Interview Questions

How do you filter data using multiple conditions?
How do you handle NULL values in the WHERE clause?
How can you use pattern matching in WHERE conditions?
What operators can you use in WHERE clauses?

SQL Comparison Operators for WHERE

Basic SQL Comparison Operators

SQL provides several operators to compare values in WHERE clauses.

SQL Logical Operators

Combine multiple conditions using logical operators.

SQL BETWEEN Operator

Test if a value falls within a range (inclusive).

SQL IN Operator

Test if a value matches any value in a list.

SQL LIKE Operator for Pattern Matching

Used for pattern matching with wildcards.

SQL NULL Value Handling

Test for NULL values (absence of data).

Note: You cannot use the = operator with NULL values. Always use IS NULL or IS NOT NULL.

Practical SQL WHERE Clause Examples

Example 1: Filtering Videos by Views and Upload Date

This example shows how to find popular recent videos:

Example 2: Complex Filtering with Multiple Conditions

This example demonstrates how to use multiple conditions to filter data:

Best Practices for SQL WHERE Clauses

1. Keep Conditions Simple

When learning SQL, start with simple conditions before combining multiple conditions. This helps build a strong understanding of how filtering works.

2. Be Careful with LIKE Patterns

Using wildcards at the beginning of a LIKE pattern (e.g., LIKE '%text') can make queries run slower. Try to use patterns that start with a known character (e.g., LIKE 'A%') when possible.

3. Use Clear and Readable Conditions

Write your WHERE conditions in a clear and readable way. Use parentheses to group conditions and make the logic easier to understand.

4. Always Use Parentheses with OR Conditions

When combining OR with AND conditions, always use parentheses to explicitly define the order of operations. This prevents logic errors and makes the query intention clear. For example, write:

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