> Do any of PostgreSQL, MySQL, SQL Server, or Oracle support this?
For Postgres:
[4.1.3. Operators](https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-OPERATORS)
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4.1.3. Operators
An operator name is a sequence of up to NAMEDATALEN-1 (63 by default) characters from the following list:
+ - * / < > = ~ ! @ # % ^ & | ` ?
There are a few restrictions on operator names, however:
-- and /* cannot appear anywhere in an operator name, since they will be taken as the start of a comment.
A multiple-character operator name cannot end in + or -, unless the name also contains at least one of these characters:
~ ! @ # % ^ & | ` ?
For example, @- is an allowed operator name, but *- is not. This restriction allows PostgreSQL to parse SQL-compliant queries without requiring spaces between tokens.
When working with non-SQL-standard operator names, you will usually need to separate adjacent operators with spaces to avoid ambiguity. For example, if you have defined a left unary operator named @, you cannot write X*@Y; you must write X* @Y to ensure that PostgreSQL reads it as two operator names not one.
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