In the programming world, I've often seen and used the phrase:
Create new table if not already exists.
Is this valid English wording and/or grammar?
The clause if not already exists is programming jargon found primarily in inline documentation or reference to it. In the interests of economy, it violates the syntax of standard English as if the words in bold if it doesn’t already exist were mere adjuncts that could be omitted. The original expression, as Laurel pointed out in a comment below, was the even more truncated
IF NOT EXISTSin MySQL, language used not between people, but people and computers.
If selected and if not already exists, a document set will be created with name as specified in the value of the specified Property column for each document item.
Jargon is a subset of a standard language created and used by a discourse group engaged in an activity or profession — sports, high finance, investment banking, the military — where technical terms may be coined whose meaning may be completely opaque to those outside the group until it is promulgated by print, broadcast, or digital media. For instance, during the Financial Crisis many, unfortunately, discovered what it meant to have an underwater mortgage.
Computer jargon even has its own dictionary, the Jargon File, which as all dictionaries will have a certain prescriptive and standardizing effect, in this case including lexical and semantic features distinct from the standard language.
Who first coined a word or expression — or at least where it first appeared — is apparently part of programming lore. This would suggest a certain status accrues to those who provide the discourse group with economical or mildly amusing coinages such as kludge, which in turn suggests one reason why this particular group is so productive in inventing new words.
In programming contexts, the use of if not already exists is standard. If at home a programmer says:
Hey, I can’t find that jar of black olives for the salad. Can you run to the store if not already exists?
then I’d begin to worry.