Basic Statements: SELECT, INSERT, UPDATE, DELETE

These four SQL statements form the foundation of everyday database operations. Mastering them is essential for any SQL professional.

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SELECT

The SELECT statement retrieves data from one or more tables.

Basic Syntax:

SELECT column1, column2
FROM table_name
WHERE condition
ORDER BY column1 ASC;

Example:

SELECT first_name, last_name
FROM employees
WHERE department = 'Sales'
ORDER BY last_name;

Key Points:

• SELECT * retrieves all columns.
• WHERE filters rows.
• ORDER BY sorts the result set.
• Use JOIN to combine tables.

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INSERT

The INSERT statement adds new rows of data to a table.

Basic Syntax:

INSERT INTO table_name (column1, column2)
VALUES (value1, value2);

Example:

INSERT INTO employees (first_name, last_name, department)
VALUES ('Alice', 'Smith', 'Marketing');

Key Points:

• You must match the order of columns and values.
• Some columns can have default values or auto-increment.
• Use INSERT INTO ... SELECT to insert data from another table.

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UPDATE

The UPDATE statement modifies existing data in a table.

Basic Syntax:

UPDATE table_name
SET column1 = value1, column2 = value2
WHERE condition;

Example:

UPDATE employees
SET department = 'HR'
WHERE employee_id = 1001;

Key Points:

• Always use WHERE to avoid updating every row.
• Multiple columns can be updated in a single statement.
• Transactions can be used to safely update large sets.

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DELETE

The DELETE statement removes rows from a table.

Basic Syntax:

DELETE FROM table_name
WHERE condition;

Example:

DELETE FROM employees
WHERE department = 'Temporary';

Key Points:

• Always use WHERE with DELETE to avoid deleting all rows.
• TRUNCATE removes all rows but is faster and cannot be rolled back in some systems.
• CASCADE may be needed to delete related rows when foreign keys are involved.

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Summary

Statement	Purpose	Example Use
SELECT	Retrieve data	Get a list of employees
INSERT	Add new data	Add a new employee
UPDATE	Modify data	Change an employee’s department
DELETE	Remove data	Delete a terminated employee

Learning these statements inside out will help you read, write, and maintain data with confidence.