SQL functions and expressions are used to perform calculations and manipulate data in a database. Functions are pre-defined pieces of code that can be used to perform specific tasks, such as converting data types, formatting dates, or calculating aggregate values. Expressions are combinations of values, operators, and functions that result in a single value.

Using built-in SQL functions

There are many built-in SQL functions that can be used for a variety of purposes. Some of the most common built-in functions include:

• COUNT(): Counts the number of rows in a table or the number of values in a column.
• SUM(): Calculates the sum of all values in a column.
• AVG(): Calculates the average of all values in a column.
• MAX(): Returns the largest value in a column.
• MIN(): Returns the smallest value in a column.

String functions

String functions are used to manipulate and format strings. Some common string functions include:

• SUBSTR(): Returns a substring of a string.
• CONCAT(): Concatenates two or more strings.
• UPPER(): Converts a string to uppercase.
• LOWER(): Converts a string to lowercase.
• TRIM(): Removes leading and trailing whitespace from a string.

Numeric functions

Numeric functions are used to perform mathematical operations on numeric values. Some common numeric functions include:

• ABS(): Returns the absolute value of a number.
• ROUND(): Rounds a number to the nearest specified number of decimal places.
• MOD(): Returns the remainder of a division operation.
• FLOOR(): Returns the largest integer less than or equal to a number.
• CEIL(): Returns the smallest integer greater than or equal to a number.

Date functions

Date functions are used to manipulate and format dates. Some common date functions include:

• SYSDATE(): Returns the current date and time.
• ADD_MONTHS(): Adds a specified number of months to a date.
• SUB_MONTHS(): Subtracts a specified number of months from a date.
• TO_CHAR(): Converts a date to a string in a specified format.
• FROM_CHAR(): Converts a string to a date in a specified format.

Combining functions in expressions

SQL functions can be combined in expressions to perform complex calculations. For example, the following expression calculates the total sales for each customer:

SELECT customer_id, SUM(sales) AS total_sales
FROM orders
GROUP BY customer_id

Expression to calculates the average order value for each customer:

SELECT customer_id, AVG(order_total) AS avg_order_value
FROM orders
GROUP BY customer_id

These are just a few examples of how SQL functions and expressions can be used to manipulate and analyze data.

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The image you sent shows the following examples of SQL functions and expressions:

Counting the number of employees in each department:

SELECT department, COUNT(*) AS num_employees
FROM employees
GROUP BY department

Calculating the sum of sales for each product:

SELECT product_name, SUM(sales) AS total_sales
FROM orders
GROUP BY product_name

Calculating the average salary for each employee:

SELECT employee_name, AVG(salary) AS avg_salary
FROM employees
GROUP BY employee_name

Finding the maximum and minimum salaries for all employees:

SELECT MAX(salary) AS max_salary, MIN(salary) AS min_salary
FROM employees

Concatenating the first and last names of each employee:

SELECT CONCAT(first_name, ' ', last_name) AS full_name
FROM employees

Converting the dates in the order_date column to the MM/DD/YYYY format:

SELECT TO_CHAR(order_date, 'MM/DD/YYYY') AS order_date
FROM orders

These are just a few examples of how SQL functions and expressions can be used to manipulate and analyze data.

Conclusion

SQL functions and expressions are a powerful tool for manipulating data. By understanding how to use functions and expressions, you can write SQL queries that are more efficient and effective.

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