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How to Automatically Generate ER Diagrams from SELECT Statements【JOIN Analysis】

Introduction

This article explains how to create ER diagrams from SQL SELECT statements.

Even when JOINs are complex and table structures are difficult to understand, you can visualize relationships using only SELECT statements.

What is an ER Diagram?

An ER diagram is a visual representation of a database's structure.

• Entity (Table)
• Attribute (Column)
• Relationship (Relation)

By representing these elements in a diagram, you can intuitively understand the structure of the data.

Benefits of Generating ER Diagrams from SELECT Statements

Understand the Overall Structure

As the number of joined tables increases, it becomes difficult to understand the structure using only SQL. Having an ER diagram allows you to grasp the overall structure at a glance.

Easier Maintenance and Modification

When modifying an existing system, having an ER diagram makes it easier to identify the impact range and reduce work costs.

No Need for Table Definitions

You can visualize the structure using only the SELECT statements in front of you, without the need to search for table definitions.

Easy for Everyone to Understand

By visualizing the structure, even non-engineers can understand it easily, which is helpful for onboarding new team members.

Why Can ER Diagrams Be Created from SELECT Statements?

Typically, ER diagrams are created from DDL statements like CREATE TABLE, but it is also possible to infer relationships between tables from SELECT statements.

The reason is that SELECT statements explicitly specify the join conditions (relationships) between tables.

For example, consider the following SQL:

SELECT *
FROM users u
JOIN orders o ON u.id = o.user_id;

In this case,

u.id = o.user_id

is the condition specified.

This represents

• users table's id column (primary key)
• orders table's user_id column (foreign key)

represents the relationship between the tables.

In other words, JOIN + ON clause = ER diagram line.

By analyzing the join conditions contained in the SELECT statement, it is possible to automatically generate an ER diagram.

How JOIN Works

JOIN is an SQL construct used to combine multiple tables and retrieve data.
By specifying join conditions, only data that matches the conditions is combined.

The basic syntax is as follows:

SELECT *
FROM TableA
JOIN TableB ON condition;

The "condition" defines the relationship between the tables.

Types of JOINs

INNER JOIN

SELECT *
FROM users u
INNER JOIN orders o ON u.id = o.user_id;

• Retrieves only the data that exists in both tables.
• The most common type of JOIN.

LEFT JOIN

SELECT *
FROM users u
LEFT JOIN orders o ON u.id = o.user_id;

• All rows from the left table (users) are included.
• Rows from the right table (orders) that do not match will be NULL.

RIGHT JOIN

SELECT *
FROM users u
RIGHT JOIN orders o ON u.id = o.user_id;

• All rows from the right table (orders) are included.
• Rows from the left table (users) that do not match will be NULL.

Relationship with ER Diagrams

JOIN is not just for data retrieval; it represents how tables are connected.

Therefore, by analyzing JOINs,

👉 it is possible to reconstruct the relationships between tables (ER diagrams)

in other words, JOINs can be used to generate ER diagrams.

How to Read the ON Clause (Most Important)

The most important aspect of ER diagram generation is analyzing the ON clause.
The ON clause directly specifies the relationships between tables.

Basic Pattern

u.id = o.user_id

From this line, we can infer the following:

• users.id is related to orders.user_id
• users is the parent table
• orders is the child table

Tips for Reading

• ① Look at both sides of the "="

👉 Each side is a column from a different table

• ② The side with "id" is often the parent

users.id ← parent
orders.user_id ← child

• ③ Multiple JOINs follow the same pattern

JOIN order_items oi ON o.id = oi.order_id

JOIN products p ON oi.product_id = p.id;

👉 You can analyze them one by one

Points to note

• Be careful with aliases (u, o, etc.)
• There may be multiple conditions (AND)
• ON o.id = s.order_id AND o.user_id = s.user_id

👉 Analyze each condition separately

Common Issues

Have you ever encountered this...?

• Too many JOINs make the structure hard to understand
• It's difficult to track table relationships in your head
• You want an ER diagram, but it doesn't exist

Solution

Utilize Automatic Generation Tools

These issues can be resolved by using tools that automatically generate ER diagrams from SELECT statements.

Our site provides a tool that can automatically generate ER diagrams from SQL.
👉 The ER diagram generation tool is here

Features

• Works entirely in the browser (no installation required)
• Frontend-only solution
• SQL input is not sent externally

Use Cases

• Analyzing existing systems
• Visualizing and sharing structures
• SQL review
• Onboarding new team members

Summary

• SELECT statements contain relationship information
• JOIN defines the relationships between tables
• By analyzing the ON clause, ER diagrams can be generated

By utilizing this mechanism, it is possible to visualize table structures even without DDL.

Using SQL2ER, these can be automatically visualized
👉 Try it out