Concat in SQL: Joining Strings

In the world of SQL, dealing with data often means going beyond crunching numbers — sometimes, you’re managing and manipulating text too. One of the most useful operations when working with strings is concatenation, or the act of joining two or more text strings together. Whether it’s formatting names, building dynamic queries, or creating descriptive labels, the ability to combine strings efficiently is essential in SQL-based data management.

TL;DR

SQL concatenation is a powerful method for joining one or more strings in your database queries. The most common way to concatenate in SQL is using the CONCAT() function, though some platforms also allow the || operator or the + symbol. Different databases support string concatenation differently, so understanding your platform is key. Use concatenation for combining names, formatting results, building dynamic commands, and more.

What Does Concatenation Mean in SQL?

Concatenation is simply the process of combining two or more strings into one. In SQL, this is useful when you want to:

• Join first and last names into a full name
• Build custom messages or derived fields
• Format output data (e.g., date labels, addresses)
• Create dynamic query elements or keys

Concatenating strings helps make your data more readable and meaningful, especially when presenting it in reports or dashboards.

Using CONCAT() in SQL

Most modern SQL dialects provide a built-in function called CONCAT() that is used to join multiple values into a single string. Here’s a basic example:

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

The above query takes the first_name and last_name columns, adds a space between them, and returns a new string as full_name.

This is especially useful when you’re selecting or displaying user-friendly text in interfaces or exports.

Concatenating More Than Two Strings

One advantage of using CONCAT() is that it supports more than just two arguments. For example:

SELECT CONCAT(title, ': ', first_name, ' ', last_name) AS formal_title
FROM employees;

This query produces a full name prefixed with the person’s title. Imagine getting the result “Dr: Jane Smith” from the respective columns. It’s a simple, elegant way to format text on the fly.

Dealing with NULL Values

One of the best features of the CONCAT() function is its handling of NULL values. Unlike some string operations that result in NULL when any argument is NULL, the CONCAT() function treats nulls as empty strings.

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

If middle_name is NULL, the query still returns a valid string without breaking. You may get “John Doe” with a double space, but it avoids producing a completely null result.

Alternative Concatenation Methods by SQL Dialect

Different SQL environments offer unique ways to perform concatenation. Let’s take a look at how it varies across some popular platforms:

1. PostgreSQL

PostgreSQL supports both the CONCAT() function and the double pipe operator ||:

SELECT first_name || ' ' || last_name AS full_name FROM employees;

This method works similarly to CONCAT() and is more concise in many cases.

2. SQL Server

SQL Server supports CONCAT() starting from SQL Server 2012. However, it also allows using the + operator:

SELECT first_name + ' ' + last_name AS full_name FROM employees;

Unlike CONCAT(), the + operator in SQL Server does not handle NULL values gracefully. If any component is NULL, the entire result is NULL. You may need to use ISNULL() or COALESCE() to work around this:

SELECT ISNULL(first_name, '') + ' ' + ISNULL(last_name, '') AS full_name FROM employees;

3. MySQL

MySQL uses CONCAT() predominantly. It also supports CONCAT_WS() (“WS” stands for “with separator”), which is very handy:

SELECT CONCAT_WS(' ', first_name, middle_name, last_name) AS full_name
FROM employees;

This version of CONCAT() inserts the separator only between non-null values, which prevents the dreaded double spaces or missing separators.

Common Use Cases

String concatenation isn’t just for showing names. It’s used in a variety of contexts:

• Creating unique keys: Combine multiple columns to generate a composite identifier.
• Building dynamic SQL queries: Assemble query strings programmatically (more common in stored procedures).
• Custom labels: Generate descriptive labels from raw data (e.g., “Region: North | ID: 00457”).
• Formatting: Organize values like addresses, dates, or other readable items.

Best Practices

While concatenation can be extraordinarily helpful, it’s important to follow some best practices to ensure your output is accurate and manageable.

• Don’t hard-code gaps or symbols directly: Use functions that support separators like CONCAT_WS() for cleaner code.
• Watch out for NULL values: Know how your SQL dialect handles NULL when joining strings.
• Use aliases: When building complex strings, always use an alias to label the result with something meaningful.
• Simplify with views or CTEs: For very long or repetitive string-building queries, use views or common table expressions to keep things readable.

Performance Considerations

Concatenation is generally not a heavy operation, but in large-scale databases, combining massive strings across millions of rows can affect performance. Also, indexes often do not apply when you’re creating strings from multiple columns dynamically, so these derived values may not be used efficiently in WHERE clauses.

If performance is a concern, consider pre-computing concatenated values and storing them in indexed columns or using materialized views.

Real-World Scenario

Imagine a customer relationship management (CRM) system where you need to display contact names with email addresses for a marketing dashboard:

SELECT CONCAT(first_name, ' ', last_name, ' <', email, '>') AS contact_info
FROM customers;

This provides outputs like “Sophie Lee <sophie.lee@example.com>”, which are perfect for user-friendly interfaces or mail merge programs.

Final Thoughts

Mastering string concatenation in SQL unlocks a world of possibilities for formatting, labeling, and dynamically manipulating your data. Whether you’re building dashboards, cleaning data, or preparing exports, knowing how to join strings correctly sets you apart as a well-rounded SQL user.

Every database comes with its own concatenation quirks, so take the time to understand the functions and operators available in your system. It’s a small skill with a big impact.

Keep your queries readable, your strings clean, and your NULLs under control!