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Filtering data in SQL involves narrowing down large datasets into smaller, more manageable subsets based on specific criteria. This process is essential for data analysts to generate meaningful insights and optimize query performance. The primary tool for filtering data in SQL is the WHERE clause, which allows you to specify conditions that the data must meet to be included in the query results.
The WHERE clause in SQL is used to filter records in database queries by specifying conditions that field values must meet to be included in the query results. The syntax for a WHERE clause is a list of conditions separated by the AND or OR keyword, depending on how the conditions are evaluated.
For example, to return records where both condition1 and condition2 must be true, the AND operator would be used.
SELECT * FROM users WHERE age >= 50;
This query selects only users who are 50 or older.
SELECT * FROM employees WHERE gender = 'female';
This query extracts records of all female employees in a company.
SELECT * FROM products WHERE genre_id = 1 AND unit_price < 1;
This query selects rows that have a genre ID equal to one and a unit price less than one.
In SQL, the WHERE clause can include multiple conditions using logical operators like AND and OR. Parentheses can also be used to prioritize and group conditions.
WHERE WORKDEPT = 'D21' AND HIREDATE > '1987-12-31'
.SQL has many logical operators that can be used to set complex conditions, including:
In SQL, the LIKE operator is used to filter data based on patterns within a specified column. It's used in conjunction with the WHERE clause to determine if a pattern matches specific values.
SELECT * FROM employees WHERE name LIKE '[JK]%';
This query finds names starting with "J" or "K".
SELECT product_id, manufacturer, drug FROM pharmacy_sales WHERE drug LIKE '%Relief%';
This query finds drugs with "Relief" in the name.
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