What does SQL Select symbol || mean?

A deeper dive into || and its behavior

Cross-dialect considerations

From the coral reefs of MySQL to the rocky mountains of ANSI SQL compliance, || operates as a chameleon, changing its behavior in different SQL environments. In MySQL, for instance, || might be interpreted as a logical OR (talk about an identity crisis!) depending on certain SQL modes. Hence, knowing your SQL environment is like knowing your Pokemon - gotta catch 'em all!

Let's talk dialects

SQL Server is that guy at the party who refuses to do the Macarena like everyone else, opting instead for its own moves. Here, the plus (+) operator does the cha-cha slide of concatenation:

SELECT first_name + ' ' + last_name AS full_name FROM users;
-- Find full name the SQL Server way: "+ is the new black"
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Oracle, on the other hand, like grandma at thanksgiving, only accepts a pair of items at a time for its CONCAT function. If you have more than two texts to concatenate, you'll need to nest them like Matryoshka dolls or stick to the || party:

SELECT CONCAT(first_name, CONCAT(' ', last_name)) AS full_name FROM users; -- Using CONCAT aka Oracle’s way or the highway
-- OR
SELECT first_name || ' ' || last_name AS full_name FROM users; -- Using ||, the easy way out
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Compatibility: Can we all get along?

Writing cross-dialect compatible SQL queries is like trying to blend in at both a Star Trek and Star Wars convention. Using functions like CONCAT(), which translate well in many SQL galaxies, you can make your command understood in even the most obscure corners of the SQL universe.

The Caped Crusader of types

The || operator moonlights as a superhero, with the special ability to concatenate any types convertible to strings. Just remember, with great power comes great responsibility:

SELECT 'Number: ' || 123 AS string_output FROM dual; -- Output: 'Number: 123'
-- "I'm not saying || is Batman, but you ever see them in the same room together?"
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Schema Variations and the Art of Readability

The || operator allows you to smoothly format your query outputs like a charming narrator without rewriting the original plot (aka your database structure):

SELECT 'Customer: ' || name || '; Total: $' || CAST(balance AS VARCHAR(10)) AS account_summary
FROM customers;
/* Never trust a computer you can't throw out a window. In other news, here's your formatted summary! */
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When to use || and when to throw it to the sharks

A note about NULL

SQL practices the philosophy of NULL in, NULL out. A single NULL string in your concatenation can result in the entire operation returning NULL. To avoid the NULL void, use functions like NVL or COALESCE:

SELECT first_name || ' ' || COALESCE(middle_name, '') || ' ' || last_name AS full_name
FROM users;
/* NULL ain't no Voldemort. We mention it here! */
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Performance: Can SQL skip leg day?

Concatenation operations are linear and usually efficient, but like anyone after Thanksgiving dinner, they can become slow and sluggish in large, overindulgent queries. Practice moderation!

Safety first!

While concatenating user input, beware of SQL injection. Always use parameterized queries or stored procedures to avoid marrying your queries with malicious input:

SELECT first_name || ' ' -- Careful, don't let your queries fall to the dark side!
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