Question

I am trying to determine that standard SQL behaviour for comparing a number to a character or string version of the same number. Does SELECT 1 = '1' (or the like) always return some sort of “truthy” value (true, 1, 't', etc.)? I have confirmed as much on PostgreSQL and MySQL, but I cannot find a resource for SQL as a whole.

Update: The purpose for the question is that I’m trying to figure out if using a number, without the quotes, will work when selecting/inserting/updating/etc. from a non-numeric field whose value is a number.

Answer 1

SELECT 1='1' gives TRUE since '1' is a correct constructor for INT in all implementation known to me.

But SQL uses strict typing, see that:

# SELECT 1=CAST('1' AS TEXT);
ERROR:  operator does not exist: integer = text
LINE 1: SELECT 1=CAST('1' AS TEXT);
                ^
HINT:  No operator matches the given name and argument type(s). You might need to add  explicit type casts.

Regarding the standard (SQL 92, 99 & 2003) it seems to be wrong:

     <literal> ::=
            <signed numeric literal>
          | <general literal>

     <general literal> ::=
            <character string literal>
          | <national character string literal>
          | <bit string literal>
          | <hex string literal>
          | <datetime literal>
          | <interval literal>

     <signed numeric literal> ::=
          [ <sign> ] <unsigned numeric literal>

     <unsigned numeric literal> ::=
            <exact numeric literal>
          | <approximate numeric literal>

     <exact numeric literal> ::=
            <unsigned integer> [ <period> [ <unsigned integer> ] ]
          | <period> <unsigned integer>

     <unsigned integer> ::= <digit>...

     <character string literal> ::=
          [ <introducer><character set specification> ]
          <quote> [ <character representation>... ] <quote>
            [ { <separator>... <quote> [ <character representation>... ] <quote> }... ]

because <quote> is only contained in <bit string literal>, <hex string literal>, … but not in numeric literals…