Thank you very much, Igor. I knew there would be a simple explanation, but
I was focused on operand affinities instead of order of operations. So, I
have to be careful about combining mathematical expressions with the
concatenation operator. I am so used to thinking of exponentiation,
multiplication and division having higher precedence than anything else in
an expression (PEMDAS) that I got blind-sided by the fact that
concatenation has even higher precedence than those. I will use
parentheses more liberally in the future to specify exactly what I want.
Thank you again.
> To: [hidden email] > Cc:
> Date: Sat, 11 Nov 2017 21:17:24 -0500
> Subject: Re: [sqlite] Can someone explain these outputs for me?
> On 11/11/2017 8:55 PM, Balaji Ramanathan wrote:
>> 3. When there is a mathematical expression after the string, I get a 0.
>> My string is nowhere to be seen in the output
>> SQLite> select '- '||cast(-1.5 as integer)*-1
> || has the highest precedence. Your expression is interpreted as ( '-
> '||cast(-1.5 as integer) ) * -1 . The string produced by the stuff in
> parentheses doesn't look like a valid number, and so becomes 0 when coerced
> to the same. Basically, you are doing
> select 'foobar' * -1
> 4. But when I add 1 instead of multiplying, it produces output that
>> seems to evaluate everything before the addition to zero
>> SQLite> select '- '||cast(-1.5 as integer)+1
> 0 * -1 == 0
> 0 + 1 == 1
> 5. Enclosing the mathematical expression in a printf produces the
>> correct output
>> SQLite> select '- '|| printf(cast(-1.5 as integer)*-1)
>> - 1
> So would enclosing in parentheses. The point is not printf() call, but
> changing the order of evaluation.
> 6. If the output starts with a number, then it doesn't seem to matter
>> what follows. Notice that the last part of the expression below is the
>> same as the expression in query number 3 above, but it works fine now
>> whereas previously it produced a zero as the output
>> SQLite> select cast(1.5 as integer)||'-'||(cast(-1.5 as integer)*-1)
> The last part is parenthesized here, whereas it wasn't in prior examples.
> That makes all the difference.
> I am sure it has something to do with order of operations and the affinity
>> of the operands, but can someone give me a summary that I can understand
>> readily? The only mentions of the "||" operator on the SQLite website (
>> https://sqlite.org/lang_expr.html) don't really explain what is going on
>> the above examples.
> The part of the article you quote that you seem to overlook is "in order
> from highest to lowest precedence"
> Igor Tandetnik