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SQL SELECT Statement
Basic SELECT Query:
The SELECT statement is used to query the database and retrieve data from one or more tables. It’s the most commonly used SQL command.
SELECT column1, column2 FROM table_name;
Example:
Retrieve the first name and last name of all employees:
SELECT first_name, last_name FROM employees;
SQL WHERE Clause
Filtering Data:
The WHERE clause is used to filter records. It is used to extract only those records that fulfill a specified condition.
SELECT column1, column2 FROM table_name WHERE condition;
Example:
Find all employees in the 'Sales' department:
SELECT * FROM employees WHERE department = 'Sales';
SQL INSERT INTO Statement
Adding New Records:
The INSERT INTO statement is used to add new rows of data to a table.
INSERT INTO table_name (column1, column2, column3) VALUES (value1, value2, value3);
Example:
Add a new employee record:
INSERT INTO employees (first_name, last_name, department) VALUES ('John', 'Doe', 'Marketing');
SQL UPDATE Statement
Modifying Existing Records:
The UPDATE statement is used to modify existing records in a table.
UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;
Example:
Update the department of an employee:
UPDATE employees SET department = 'HR' WHERE employee_id = 101;
SQL DELETE Statement
Removing Records:
The DELETE statement is used to remove existing records from a table.
DELETE FROM table_name WHERE condition;
Example:
Delete an employee record:
DELETE FROM employees WHERE employee_id = 102;
SQL JOIN Clause
Combining Tables:
The JOIN clause is used to combine rows from two or more tables, based on a related column between them.
SELECT columns FROM table1 INNER JOIN table2 ON table1.column = table2.column;
Example:
Join employees and departments tables to get employee names with their department names:
SELECT employees.first_name, departments.department_name
FROM employees
INNER JOIN departments ON employees.department_id = departments.id;
SQL GROUP BY Clause
Aggregating Data:
The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country".
SELECT column_name, COUNT(*)
FROM table_name
GROUP BY column_name;
Example:
Count the number of employees in each department:
SELECT department, COUNT(*)
FROM employees
GROUP BY department;
SQL ORDER BY Clause
Sorting Data:
The ORDER BY keyword is used to sort the result-set in ascending or descending order.
SELECT column1, column2 FROM table_name ORDER BY column1 ASC|DESC;
Example:
List all employees ordered by their last name:
SELECT first_name, last_name FROM employees ORDER BY last_name ASC;
