Straight forward guide on MYSQL database design.The importance of MYSQL database design cannot be overstated. Good design ensures data organization is optimized for efficiency and quick data retrieval.
In this article, we will look at the basics of design and cover essential topics such as executing multiple joins in one query, creating and populating tables, and foreign keys. Without further ado, let’s dive in.
Three-Table Join in One Query
In some instances, it might be necessary to combine data from multiple tables. A three-table join is a way of accomplishing this.
We shall look at three ways to execute a three-table join, which are:
1.
Three-Table Join With a Natural Join
The natural join enables us to execute a three-table join without using the ON keyword.
The query’s database engine will determine the join conditions based on the column names from each table that match. Example:
SELECT * from orders natural JOIN clients natural Join order_items;
2.
Three-Table Join With the ON Keyword
The ON keyword specifies the join condition, making it possible to execute a three-table join by explicitly stating the relation between tables. Example:
SELECT * from clients JOIN orders on clients.client_id=orders.client_id JOIN order_items on order_items.order_id=orders.order_id;
3.
Three-Table Join Within WHERE Block
This approach involves selecting table data twice, and the second selection is within the WHERE block. This method is not as elegant as the earlier two but is still effective.
Example:
SELECT * from order_items, orders, clients where orders.client_id=clients.client_id and order_items.order_id=orders.order_id;
The Outer Join Case
Outer join returns all the columns from one table, and the matching columns for the other table. It is useful for when it is necessary to obtain data from one table even when a corresponding entry is not in the other table.
Example:
SELECT * from clients LEFT OUTER JOIN orders on clients.client_id=orders.client_id;
Creating and Populating Tables
In MYSQL, creating a table involves defining the table schema, and populating it involves adding data. Below we have two tables to illustrate this:
1.
Product Table
The product table is responsible for storing product data. Example:
CREATE TABLE products(
product_id INT (11) PRIMARY KEY,
product_name VARCHAR (128),
product_desc VARCHAR (128),
product_price FLOAT(4),
date_created TIMESTAMP,
date_updated TIMESTAMP
);
2. Product_Order Table
The Product_Order table is responsible for storing a shopping cart or order.
Example:
CREATE TABLE product_order(
order_id INT (11) PRIMARY KEY,
order_desc VARCHAR (128),
order_total FLOAT(4),
date_created TIMESTAMP,
date_updated TIMESTAMP
);
Foreign Keys
A foreign key is a database entity’s reference to another database entity’s primary key. It is a way of reinforcing the relationship between tables.
Example:
CREATE TABLE products (
product_id INT (11) PRIMARY KEY,
product_name VARCHAR (128),
product_desc VARCHAR (128),
product_price FLOAT(4),
date_created TIMESTAMP,
date_updated TIMESTAMP,
order_id INT(11),
FOREIGN KEY (order_id) REFERENCES product_order(order_id)
ON DELETE CASCADE
ON UPDATE CASCADE
);
Conclusion:
In summary, we have looked at executing multiple joins in one query through three different methods: natural join, ON keyword, and WHERE block. Then we delved into creating and populating tables, including the product table and product_order table, and the significance of foreign keys.
These are the foundational concepts any programmer or web developer should understand when designing databases in MySQL. Expanding your Knowledge of MYSQL Database DesignThe MYSQL database management system is a relevant tool for managing data effectively.
MYSQL’s core task is to handle databases that can store and manage data in an organized way. In this article, we will look at calculating profit for each client using MYSQL and how to perform a three-table join via different methods.
Let’s get started. Goal of the Query: Calculating Profit for Each Client
Calculating the profit for each client is essential in ensuring a business remains profitable.
To calculate profit in MySQL, we use the following formula:
Profit = (Quantity * Unit_Price) – Supplier_Cost
Where Quantity is the number of products sold, Unit_Price is the price for each sold item, and Supplier_Cost is the cost to the supplier.
Three-Table Join
A three-table join in MySQL is like a two-table join statement but involves combining three or more tables into one query to achieve more complex results. The basic syntax for executing a three-table join in MYSQL is as follows:
SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column_name1 = table2.column_name2 INNER JOIN table3 ON table2.column_name3=table3.column_name3;
Method 1: Natural Join
A natural join is a way of linking tables by matching every column that shares identical names while retaining only one column.
It is an automatic join method that does not require any join conditions. Example:
SELECT clients.client_name, SUM(order_items.quantity*(order_items.unit_price – order_items.supplier_cost)) AS profit FROM clients NATURAL JOIN orders NATURAL JOIN order_items GROUP BY clients.client_name;
Method 2: ON Keyword
An ON keyword method is a conditional join that specifies the conditions to match columns from the different tables.
It is more versatile than the natural join method. Example:
SELECT clients.client_name, SUM(order_items.quantity*(order_items.unit_price – order_items.supplier_cost)) AS profit FROM clients JOIN orders ON clients.client_id=orders.client_id JOIN order_items ON orders.order_id=order_items.order_id GROUP BY clients.client_name;
Method 3: WHERE Block
A WHERE block joins tables through selecting table data twice.
We use the WHERE block to filter out the correct data we need from the tables being joined. Example:
SELECT clients.client_name, SUM(order_items.quantity*(order_items.unit_price – order_items.supplier_cost)) AS profit FROM order_items, orders, clients WHERE order_items.order_id=orders.order_id AND orders.client_id=clients.client_id GROUP BY clients.client_name;
Controlling Data with GROUP BY Clause
The GROUP BY clause is vital for obtaining summary information about specific file groups in a table and used to separate data into groups according to specified columns. The GROUP BY clause is used to aggregate data.
In calculating a client’s profit, we use the GROUP BY clause to aggregate the data according to client_name. Example:
SELECT clients.client_name, SUM(order_items.quantity*(order_items.unit_price – order_items.supplier_cost)) AS profit FROM clients JOIN orders ON clients.client_id=orders.client_id JOIN order_items ON orders.order_id=order_items.order_id GROUP BY clients.client_name;
Conclusion:
In conclusion, we’ve looked at the goal of calculating profit for each client by utilizing the profit calculation formula in MYSQL.
We also learned different methods of performing a three-table join in MYSQL, such as natural join, ON keyword, and WHERE block. Lastly, we reviewed how the GROUP BY clause controls data to obtain a summary of specific file groups in a table.
With these concepts, MYSQL database designers have some powerful tools at their disposal for designing and managing databases efficiently. MySQL Database Design: Outer JoiningA relational database stores data in a table structure using logical relationships between tables.
SQL programming is the standard method used for working with relational databases such as MySQL. In MySQL, the JOIN command is used to merge data from two or more tables.
This article focuses on the problems with using inner joins, introduces the solution of using outer joins, and explores how null values affect outer joins. Problem with Inner Join:
The INNER JOIN selects only the matching records from both tables.
It is the most widely used join type, and it only returns rows from both tables when the values in the related columns match. A problem arises when there is missing data in either of the tables.
For example, suppose you have two tables of sales data, the Sales Rep table, and the Sales Table. An inner join of the two tables would only return data where there is a matching sales rep name between the two tables.
This means that you would not get any data for the sales reps that were not present in the Sales Table. Example:
SELECT Sales_Rep.Name, Sales_Table.Product, Sales_Table.Sale_Amount
FROM Sales_Rep INNER JOIN Sales_Table
ON Sales_Rep.ID = Sales_Table.Rep_ID
ORDER BY Sales_Rep.Name;
Solution with Outer Join:
An outer join is a solution for the problem of missing data when using inner joins. It returns all the rows from both tables and substitutes NULL values for the missing data.
Example:
SELECT Sales_Rep.Name, Sales_Table.Product, Sales_Table.Sale_Amount
FROM Sales_Rep LEFT OUTER JOIN Sales_Table
ON Sales_Rep.ID = Sales_Table.Rep_ID
ORDER BY Sales_Rep.Name;
In the above example, we use LEFT OUTER JOIN to return all the rows in the Sales_Rep table, including the sales reps for whom there are no sales records, and NULL values are returned for the sales column fields in the Sales_ Table. The LEFT OUTER JOIN will display all rows from the Sales Reps table and matching rows from the Sales Table.
One can also use a RIGHT OUTER JOIN that will display all the rows from the Sales Table and matching rows from the Sales Reps table. Usage of Null Values in Outer Joins:
In an outer join, null values are used to signify missing data.
The IFNULL function is used to replace null values with default values. The syntax for the IFNULL function is:
IFNULL(expression, default_value);
The IFNULL function evaluates the expression and returns the result if it is not null.
If the expression is null, the function returns the default_value. Example:
SELECT Sales_Rep.Name, IFNULL(Sales_Table.Product, “Not Available”) , IFNULL(Sales_Table.Sale_Amount, 0)
FROM Sales_Rep LEFT OUTER JOIN Sales_Table
ON Sales_Rep.ID = Sales_Table.Rep_ID
ORDER BY Sales_Rep.Name;
In the above example, we use IFNULL function to replace null values with “Not Available” for products and 0 for sale_amount. This makes it easy to read and compare values.
Conclusion:
In conclusion, inner joins can lead to incomplete data display and create confusion. Outer joins solve this problem by returning all the rows from both tables and substituting nulls for any missing data.
Additionally, we can use the IFNULL function to replace null values with default values. Making use of outer join in MySQL is essential in ensuring the completeness of data, and one needs to understand the syntax and its functionality.
In summary, this article explored the problem with using inner joins in MYSQL, which can lead to incomplete data displays and provided a solution through outer joins. Outer joins return all rows from both tables and substitute nulls for missing data, and we can use the IFNULL function to replace null values with default values.
Understanding outer joins is crucial in ensuring data completeness and database efficiency in MYSQL. Takeaway, outer joining facilitates complete data displays, essential to avoid incomplete data or business decisions based on incorrect information.
Lastly, making use of the IFNULL function, guarantees readability, and helps compare values accurately.