What does SQL Select symbol || mean?

问题

What does || do in SQL?

SELECT 'a' || ',' || 'b' AS letter

回答1:

|| represents string concatenation. Unfortunately, string concatenation is not completely portable across all sql dialects:

ansi sql: || (infix operator)
mysql: concat ( vararg function ). caution: || means 'logical or' (It's configurable, however; thanks to @hvd for pointing that out)
oracle: || (infix operator), concat ( caution: function of arity 2 only ! )
postgres: || (infix operator)
sql server: + (infix operator), concat ( vararg function )
sqlite: || (infix operator)

hopefully the confusion is complete ...

回答2:

It is a concat statement. It will concatenate the two strings.

Here is a helpful post!

What is the difference between "||" operator and concat function in Oracle?

回答3:

SELECT 'a' || ',' || 'b' AS letter will combine a letter. The result become 'a,b'

回答4:

In Oracle, SQLite3, and MySQL, it concatenates strings. Please see the Oracle documentation. The MySQL documentation.

Also, it's part of ANSI SQL, but read this for more information.

回答5:

It's a concatenation operator. So you would get 'a,b' from that. I think || will work on most RDBMS's. SQL Server requires the + operator (thanks to HVD for setting me straight!).