Identifiers

Normal identifiers

Most dialects support [a-zA-Z_] as first character and [a-zA-Z0-9_] as rest of the characters. The differences from this are listed below:

BigQuery: single dashes (-) can be used, but not at the beginning or end.
DB2: first char can only be a (uppercase) letter (a lowercase letter gets converted to uppercase).
Hive: (no differences)
MariaDB: no first-letter restrictions. The characters [a-zA-Z0-9_$] and unicode letters are allowed everywhere. Can begin with digit, but can't only contain digits.
MySQL: same as MariaDB.
N1QL: (no differences)
PL/SQL: can't start with _. Allows $, # in rest of the identifier.
PostgreSQL: additionally $ after first char. Also unicode letters are allowed.
Redshift: also unicode letters are allowed.
Spark: Seems like the usual syntax is allowed. But the docs are confusing.
SQLite: (no differences)
Transact-SQL: @ and # are allowed as first chars plus $ in the rest. Also unicode letters are allowed. Though the beginning @ signifies a local variable or parameter and # a temporary table or procedure.
Trino: [a-zA-Z0-9_]+, no first-letter restrictions

SQL standard specifies double-quotes ".." for delimited identifiers. There is a considerable variation in implementations:

Notes: