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Editorial Team
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Asked: 2026-05-29T11:27:02+00:00

For my own interests I am writing an ANSI SQL Lexer. Specifically, I am

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For my own interests I am writing an ANSI SQL Lexer. Specifically, I am trying to conform to ISO/IEC 9075-2:2003(E). I ran into a problem in the token stage with some ambiguity.

The lexical elements section define an interval string as follows: 

<interval string> ::= <quote> <unquoted interval string> <quote>
<unquoted interval string> ::= [ <sign> ] { <year-month literal> | <day-time literal> }
<year-month literal> ::= <years value> [ <minus sign> <months value> ]  | <months value>
<years value> ::= <datetime value>
<months value> ::= <datetime value>
<datetime value> ::= <unsigned integer>
<unsigned integer> ::= <digit>...
<digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Example:
’30’

Is the 30 a <years value> without the option
or is it a <months value>?

In theory I could write:
SELECT ’30’

I created a YearsValue token and a MonthsValue token (classes). However, the ambiguity is an issue, it matches both. I don’t see anything specifically dealing with multiple matches in part 1 or part 2 of ISO/IEC 9075.

Can someone point out where in the spec this is handled or is it just assumed left to right?

Before anyone asks, I am doing this because I want to write a SQL lexer. Its not for school its just something to educate myself. I don’t want to use GOLD or ANTLR either.

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1 Answer

  1. Editorial Team

    Is the 30 a <years value> without the option or is it a <months value>?

    Based on my reading of a draft of SQL 2003, it is left ambiguous in a way that doesn’t matter. Yes, the grammar does not specify whether the 1 in INTERVAL '1' YEAR is a <years value> or a <months value>, or even perhaps a <days value>, but it really does not matter. The description of how YEAR is interpreted is clear that 1 is a number of years, even if it is parsed as a <months value>. The standard says that the first component in the value is mapped to the first field type in the interval type:

    5.3 <literal>

    General Rules

    7) The i-th datetime component in a <datetime literal> or <interval literal> assigns the value of the datetime component to the i-th <primary datetime field> in the <datetime literal> or <interval literal>.

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