DbSchema | SQLite - How to Use UNION Clause?

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Table of Contents

1. Introduction
2. Prerequisites
3. UNION Clause
4. Advantages and Limitations of UNION Clause
5. Restrictions on UNION Clause
6. Difference between UNION Clause and UNION ALL Clause
7. Implementing UNION in sqlite3
8. Implementing UNION in DbSchema
9. Conclusion
10. References

Introduction

SQLite is a widely used __relational database management system that employs SQL (Structured Query Language). This __article explores the use of the __UNION clause in SQLite, specifically within the __sqlite3 and __DbSchema environments. The __UNION operator allows the combination of two or more __SELECT` statements into a single result set, providing that each SELECT statement contains the same number of columns and similar data types.

Prerequisites

Before you proceed, ensure that you have:

1. __SQLite` installed on your machine. You can download it from the official SQLite website.
2. __DbSchema` installed. Download it from the DbSchema website.
3. Basic knowledge of __SQL` syntax.

For installation and establishing connection you can read our article SQLite-How to create a database?

UNION Clause

In SQLite, the __UNION clause __combines the result sets of two or more __SELECT statements into a single result set. The __UNION operator removes duplicate rows from the result.

Advantages and Limitations of UNION Clause

Advantages

1. It allows the __combination of results from multiple SELECT statements into a single __comprehensive result.
2. It __removes duplicate entries, ensuring that the final result is __unique.

Limitations

1. The UNION operator can be __slower compared to individual SELECT statements, especially on large datasets, because it needs to remove __duplicate entries.
2. All __SELECT statements within the __UNION must have the __same number of columns` and corresponding data types.

Restrictions on UNION Clause

1. The number of columns and their __data types must be the same in all the SELECT statements being __united.
2. The column names in the __result set are usually determined by the column names in the first __SELECT statement.

Difference between UNION Clause and UNION ALL Clause

UNION Clause	UNION ALL Clause
Removes duplicate rows from the result.	Does not remove duplicate rows from the result.
May be slower due to the process of removing duplicates.	Faster, as it does not need to process removing duplicates.

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Implementing UNION in sqlite3

Here’s how you can implement the UNION operator in sqlite3:

Step 1: Install sqlite3

If you haven’t already, you need to install sqlite3 on your machine. You can download it from the official SQLite website.

Step 2: Start sqlite3

To start sqlite3, open your terminal and type __sqlite3`. You will then see the sqlite3 command-line interface.

sqlite3  

Step 3: Create a New Database

To create a new database, type the following command:

.open sampleDB.db

To know more about creating a database you can read our article SQLite-How to create a database?

Step 4: Create the Tables and Insert Data

Next, let’s create two tables, __Employees and __Contractors, and insert some data into them.

CREATE TABLE Employees (
  id INTEGER PRIMARY KEY,
  name TEXT,
  department TEXT
);

INSERT INTO Employees (name, department) VALUES 
('John Doe', 'Sales'),
('Jane Doe', 'Marketing');

CREATE TABLE Contractors (
  id INTEGER PRIMARY KEY,
  name TEXT,
  department TEXT
);

INSERT INTO Contractors (name, department) VALUES 
('Mark Smith', 'Sales'),
('Jane Smith', 'Marketing');

To know more about creating a table you can read our article SQLite-How to create a Table?

Step 5: Implement UNION

Now that we have some data in our tables, we can implement the UNION operator.

SELECT name, department FROM Employees
 UNION
SELECT name, department FROM Contractors;

Sample Database:

Employees Table:

name	department
__John Doe`	Sales
__Jane Doe`	Marketing

Contractors Table:

name	department
__Mark Smith`	Sales
__Jane Smith`	Marketing

Results:

Following result will be obtained after executing UNION Clause:

name	department
__John Doe`	Sales
__Jane Doe`	Marketing
__Mark Smith`	Sales
__Jane Smith`	Marketing

Step 6: Implement UNION ALL

Now we will implement UNION ALL clause on our sample database.

SELECT name, department FROM Employees
UNION ALL
SELECT name, department FROM Contractors;

Results:

Following result will be obtained after executing __UNION ALL` Clause:

name	department
__John Doe`	Sales
__Jane Doe`	Marketing
__Mark Smith`	Sales
__Jane Smith`	Marketing

Notice that if there were __duplicate` records between the Employees and Contractors tables, UNION ALL would have included them in the result.

Implementing UNION in DbSchema

Step 1: Install DbSchema

If you haven’t already, you need to install DbSchema on your machine. You can download it from the official DbSchema website.

Step 2: Connect to Your SQLite Database

After you open DbSchema, you need to connect it to your SQLite database. You can do this by clicking on __Connect to database` and filling in the necessary details.

Step 3: Create the Tables and Insert Data

Next, let’s create the __Employees and __Contractors tables and insert some data into them. You can do this in the ‘SQL Editor’.

To know more about creating a table you can read our article SQLite-How to create a Table?

Step 4: Implement UNION

Finally, we can implement the UNION operator. You can do this in the __SQL Editor` as well.

SELECT name, department FROM Employees
UNION
SELECT name, department FROM Contractors;

After you run the query, you can see the results in the ‘Query Results’ tab.

Visually Manage SQLite using DbSchema

DbSchema is a __SQLite client and __visual designer. DbSchema has a free Community Edition, which can be downloaded here.

Key Features of DbSchema:

Following are the key features of DbSchema which distinguish it from other database GUI tools.

Conclusion

The __UNION operator is a powerful tool in SQL that allows you to __combine the results of multiple __SELECT statements. However, it does have certain limitations and should be used carefully. While the __implementation of the UNION operator is pretty standard across different SQL environments, the specific way to do it may vary __slightly`.

References

1. SQLite UNION Operator
2. DbSchema Documentation
3. SQLite Official Website
4. DbSchema Official Website