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Table C.1 lists all tokens that are key words in the SQL standard and inPostgres Pro Standard 17.4.1. Background information can be found inSection 4.1.1. (For space reasons, only the latest two versions of the SQL standard, and SQL-92 for historical comparison, are included. The differences between those and the other intermediate standard versions are small.)

SQL distinguishes betweenreserved andnon-reserved key words. According to the standard, reserved key words are the only real key words; they are never allowed as identifiers. Non-reserved key words only have a special meaning in particular contexts and can be used as identifiers in other contexts. Most non-reserved key words are actually the names of built-in tables and functions specified by SQL. The concept of non-reserved key words essentially only exists to declare that some predefined meaning is attached to a word in some contexts.

In thePostgres Pro parser, life is a bit more complicated. There are several different classes of tokens ranging from those that can never be used as an identifier to those that have absolutely no special status in the parser, but are considered ordinary identifiers. (The latter is usually the case for functions specified by SQL.) Even reserved key words are not completely reserved inPostgres Pro, but can be used as column labels (for example,SELECT 55 AS CHECK, even thoughCHECK is a reserved key word).

InTable C.1 in the column forPostgres Pro we classify as“non-reserved” those key words that are explicitly known to the parser but are allowed as column or table names. Some key words that are otherwise non-reserved cannot be used as function or data type names and are marked accordingly. (Most of these words represent built-in functions or data types with special syntax. The function or type is still available but it cannot be redefined by the user.) Labeled“reserved” are those tokens that are not allowed as column or table names. Some reserved key words are allowable as names for functions or data types; this is also shown in the table. If not so marked, a reserved key word is only allowed as a column label. A blank entry in this column means that the word is treated as an ordinary identifier byPostgres Pro.

Furthermore, while most key words can be used as“bare” column labels without writingAS before them (as described inSection 7.3.2), there are a few that require a leadingAS to avoid ambiguity. These are marked in the table as“requiresAS”.

As a general rule, if you get spurious parser errors for commands that use any of the listed key words as an identifier, you should try quoting the identifier to see if the problem goes away.

It is important to understand before studyingTable C.1 that the fact that a key word is not reserved inPostgres Pro does not mean that the feature related to the word is not implemented. Conversely, the presence of a key word does not indicate the existence of a feature.