SQL filtering sorting are essential skills for querying databases effectively. The clause allows you to filter rows based on specific conditions, using operators =, <>, and . You can combine conditions with AND, , and for complex queries.
Sorting results is done using the clause, which arranges data based on specified columns. You can sort ascending or , and even sort by multiple columns. Understanding NULL handling is crucial for accurate filtering and sorting in SQL.
Filtering Data
Conditional Filtering with the WHERE Clause
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Use the WHERE clause to filter rows based on specified conditions
Place the WHERE clause after the FROM clause in a statement
Specify the condition using comparison operators (=, <>, >, <, >=, <=)
Combine multiple conditions using logical operators
AND returns true if all conditions are true
OR returns true if at least one condition is true
NOT negates the result of the condition
Use parentheses to control the order of evaluation for complex conditions
Range and Membership Filtering
Filter values within a range using the BETWEEN operator
Syntax:
column_name BETWEEN value1 AND value2
Inclusive of both value1 and value2 (greater than or equal to value1 and less than or equal to value2)
Check for membership within a set of values using the IN operator
Syntax:
column_name IN (value1, value2, ...)
Equivalent to multiple OR conditions
Filter based on pattern matching using the LIKE operator
Use wildcards to represent characters
%
matches any sequence of zero or more characters
_
matches any single character
Syntax:
column_name LIKE 'pattern'
(case-sensitive) or
column_name ILIKE 'pattern'
(case-insensitive)
Examples:
name LIKE 'A%'
matches names starting with 'A'
email LIKE '%@example.com'
matches emails ending with '@example.com'
Handling NULL Values
Check for NULL values using the operator
Syntax:
column_name IS NULL
Check for non-NULL values using the operator
Syntax:
column_name IS NOT NULL
NULL represents an unknown or missing value
Comparison operators (=, <>, etc.) do not work with NULL values
Use IS NULL or IS NOT NULL to explicitly check for NULL values
Sorting Results
Ordering Query Results
Use the ORDER BY clause to sort the result set based on one or more columns
Place the ORDER BY clause after the WHERE clause (if present) in a SELECT statement
Specify the column(s) to sort by, separated by commas
Optionally, specify the sort direction for each column using ASC (ascending, default) or DESC (descending)
Syntax:
ORDER BY column1 [ASC|DESC], column2 [ASC|DESC], ...
Sorting is performed in the specified order (column1, then column2, etc.)
Sorting Order and NULL Values
NULL values are considered the lowest values when sorting in
NULL values are considered the highest values when sorting in descending order
Examples:
SELECT * FROM employees ORDER BY salary DESC
sorts employees by salary in descending order
SELECT * FROM products ORDER BY category ASC, price DESC
sorts products by category in ascending order, and within each category, sorts by price in descending order