mysql - What does SQL Select symbol || mean?

Question

What does || do in SQL?

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

Answer 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 ...