Author: admin
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JOIN Clause
Purpose: The JOIN clause combines rows from two or more tables based on a related column, facilitating complex queries across multiple tables.Types of Joins: Example Explanation: This retrieves employee names along with their department names by joining the employees and departments tables on department_id.
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HAVING Clause
Purpose: The HAVING clause filters results after aggregation, allowing you to specify conditions on grouped records.Syntax Example: Explanation: This retrieves departments where the average salary exceeds 50,000. HAVING is used because the condition is applied after grouping.
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GROUP BY Clause
Purpose: The GROUP BY clause groups rows that have the same values in specified columns into summary rows, often used with aggregate functions.Syntax: Example: Explanation: This counts the number of employees in each department, returning one row per department.
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ORDER BY Clause
Purpose: The ORDER BY clause sorts the result set based on one or more columns, in ascending or descending order.Syntax: Example: Explanation: This sorts the results alphabetically by last_name. The default is ascending order; use DESC for descending.
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WHERE Clause
Purpose: The WHERE clause filters records based on specific conditions, limiting the results to those that meet the criteria.Syntax: Example: Explanation: This retrieves all records from the employees table where the salary exceeds 60,000.
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FROM Clause
Purpose: The FROM clause identifies the table(s) from which to fetch data.Syntax: Example: Explanation: This selects all columns from the departments table. You can also join multiple tables here.
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SELECT Clause
Purpose: The SELECT clause specifies which columns or expressions to retrieve from a database.Syntax: Example: Explanation: This retrieves the first_name and last_name columns from the employees table. You can use * to select all columns.
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Deleting Data
DELETE Statement: Used to remove records from a table. Syntax: Example: Explanation: This deletes the employee record with employee_id 1 from the employees table. The WHERE clause is crucial to avoid deleting all records.
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Inserting Data
INSERT Statement: Used to add new records to a table.Syntax: Example: Explanation: This adds a new employee named Jane Doe with a salary of 70,000 in department 10. Each value corresponds to the specified column.
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Updating Data
UPDATE Statement: Used to modify existing records in a table.Syntax: Example: Explanation: This increases the salary of all employees in department 10 by 10%. The WHERE clause ensures that only the relevant records are updated.