Expressions

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Published Jun 20, 2024
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In MySQL, Expressions are combinations of one or more values, operators, and functions that evaluate to produce a result. Expressions are used in various parts of SQL queries to compute values, filter records, and perform operations. They are fundamental building blocks for constructing meaningful queries and performing calculations within the database.

Types of MySQL Expressions

These are some of the common MySQL Expressions:

Arithmetic Expressions

Arithmetic expressions involve numerical calculations and use arithmetic operators such as +, -, *, and /.

Column Expressions

Column expressions refer to the columns of a table and are used to specify or manipulate the column data.

String Expressions

String expressions involve operations on string data types, such as concatenation, manipulation, and evaluation operations. For example, the CONCAT() function is commonly used to concatenate two or more strings into one.

Aggregate Expression

Aggregate expressions perform calculations on multiple rows and return a single value. Common aggregate functions include AVG(), SUM(), COUNT(), MIN(), and MAX().

Boolean Expressions

Boolean expressions return a boolean value, typically used in WHERE clauses to filter records. For example, price > 10 is a boolean expression that could be used in SELECT * FROM products WHERE price > 10; to filter products with a price greater than 10.

Function Expressions

Function expressions use MySQL functions to perform operations on data. For example, NOW() retrieves the current timestamp.

Example

Consider the following SQL statement used to create a table with sample data:

-- Create a table named 'products'
CREATE TABLE products (
name VARCHAR(50),
price DECIMAL(10, 2),
quantity_in_stock INT
);
-- Insert data into the 'products' table
INSERT INTO products (name, price, quantity_in_stock) VALUES
('Product A', 19.99, 10),
('Product B', 5.99, 20),
('Product C', 49.99, 5);

Arithmetic Expressions

The following example calculates the total value of each product in stock:

SELECT
name,
price,
quantity_in_stock,
price * quantity_in_stock AS total_value
FROM
products;

The above query will produce the following output:

name price quantity_in_stock total_value
Product A 19.99 10 199.90
Product B 5.99 20 119.80
Product C 49.99 5 249.95

Column Expressions

The following example retrieves the name and price columns from the products table:

SELECT name, price FROM products;

The above query will produce the following output:

name price
Product A 19.99
Product B 5.99
Product C 49.99

String Expressions

In the following example, the concatenation operation CONCAT(name, ' - Special Offer') combines the name column value with the string ' - Special Offer'.

SELECT CONCAT(name, ' - Special Offer') AS special_offer FROM products;

The above query will produce the following output:

special_offer
Product A - Special Offer
Product B - Special Offer
Product C - Special Offer

Aggregate Expressions

The following example calculates the total price of all products:

SELECT SUM(price) AS total_price FROM products;

The above query will produce the following output:

total_price
75.97

Boolean Expressions

The following example retrieves products with low stock:

SELECT * FROM products WHERE quantity_in_stock < 10;

The above query will produce the following output:

name price quantity_in_stock
Product C 49.99 5

Function Expressions

The following example retrieves the length of each product name:

SELECT name, LENGTH(name) AS name_length FROM products;

The above query will produce the following output:

name name_length
Product A 9
Product B 9
Product C 9

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