# ISNUMERIC

• The discussion usually answer my questions but not this time. Doesn't "isnumeric(char(32)) as n" return 0? And if so wouldn't "select sum(n) from l" also equal 0? I guess I don't see how the sum of n got to 16 if n is alway 0?

Thanks, Jerry

• Interesting question and great feedback from the discussion!!

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• jlennartz (9/27/2010)

The discussion usually answer my questions but not this time. Doesn't "isnumeric(char(32)) as n" return 0? And if so wouldn't "select sum(n) from l" also equal 0? I guess I don't see how the sum of n got to 16 if n is alway 0?

Thanks, Jerry

Jerry, the recursive CTE [i.e, "with l as ( .... )" ] creates a set of rows with the value being tested ranging from 32 to 127, so the code actually tests each character from char(32) (i.e, ' ') to char(127). There are sixteen characters in that range that can be convertible to one or more data types (usually money, as that type seems to have a very broad tolerance for conversion.) See my earlier post for a reference to recursive CTEs.

• jlennartz (9/27/2010)

I guess I don't see how the sum of n got to 16 if n is alway 0?

Jerry, hopefully someone will post a tidy explanation of how a recursive CTE works. In the meantime, run this SQL to see the entire contents of the 'l' table:

with l as (

select 32 as i, char(32) as s, isnumeric(char(32)) as n

union all

select i+1, char(i+1), isnumeric(char(i+1)) from l where i < 127

)

select * from l

• jlennartz (9/27/2010)

The discussion usually answer my questions but not this time. Doesn't "isnumeric(char(32)) as n" return 0? And if so wouldn't "select sum(n) from l" also equal 0? I guess I don't see how the sum of n got to 16 if n is alway 0?

The start of the query is a "recursive CTE". It consists of an anchor query and a recursive query. The anchor query is:

`select 32 as i, char(32) as s, isnumeric(char(32)) as n`

This returns a single row, with columns 1 (integer value 32), s (char(1), value ' ' (the space character)), and column n (integer value 0, the result of ISNUMERIC(char(32))).

The recursive query is:

`select i+1, char(i+1), isnumeric(char(i+1)) from l where i < 127`

Note that the FROM clause references the CTE itself. That makes the CTE recursive. On the first iteration, the FROM denotes the reqults of the anchor query. Since i = 32 in that row, the recursive part will now produce a similar row for the value i+1 (33). But then the recursing starts. The recursive part is evaluated again, with this new row as input, so now a row is produced that starts with i = 33 and generates column values for the value 34. This continues until the WHERE clause is no longer satisfied.

[The actual execution plan will probably be more efficient than this!]

The result is a table with 96 rows, with i ranging from 32 to 127 and s and n representing char(i) and isnumeri(char(i)) for each row.

The outer query then sums those isnumeric values.

Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

• Thank You sknox, wware and especially Hugo. With Hugo's explaination it all became clear.

Thanks to all,

Jerry

• jlennartz (9/27/2010)

...With Hugo's explaination it all became clear.

It generally does. 😀

• Why does `select i+1, char(i+1), isnumeric(char(i+1)) from l where i < 127` need an i+1 in each column?

• foxxo (9/27/2010)

Why does `select i+1, char(i+1), isnumeric(char(i+1)) from l where i < 127` need an i+1 in each column?

If you use

`select i+1, char(i), isnumeric(char(i)) from l where i < 127`

Then you'd probably get the same results from the summation, but if you check the actual rows produced by the CTE, you'd see weird results. For example, when i = 80, you'd get the number 81 for the row with char(80) and isnumeric(char(80)).

Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

• 78% correct answers?! Yeah, right.

Great question and good points in discussion by Hugo. Pleasure to read.

But, thanks to all who participated in it.

Hrvoje

Hrvoje Piasevoli

• A good question that has taught me two things:

1. You can't trust SQL's ISNUMERIC function

2. You can build a recursive CTE - although I'm no sure I would want to 😉

If anyone else has examples of CTE's I bet they'd make for some tricky questions?

Thanks

• jts_2003 (9/29/2010)

2. You can build a recursive CTE - although I'm no sure I would want to 😉

Recursive CTE'sare an invaluable tool when working with hierarchic data (like employee/supervisor relations, parts built of parts built of parts, genealogy data, etc). There are some examples of this in Books Online.

Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

• Very nice question. And one more time I see that 'isnumeric' works in a way which is far away from our expectations. Always used custom made functions instead.

• Great question! Now that I know how "liberal" is numeric is, I will be more careful with it.

Thanks!!!

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