Sybase: HAVING operates on rows?

Question

I\'ve came across the following SYBASE SQL:

-- Setup first
create table #t (id int, ts int)
go

insert into #t values (1, 2)
insert into #t values (1, 10)
insert         


        

Answer 1

My understanding: Yes, fundamentally, HAVING operates on rows. By omitting a GROUP BY, it operates on all result rows within a single "supergroup" rather than on rows-within-groups. Read the section "How group by and having queries with aggregates work" in your originally-linked Sybase docco:-

How group by and having queries with aggregates work

• The where clause excludes rows that do not meet its search conditions; its function remains the same for grouped or nongrouped queries.
• The group by clause collects the remaining rows into one group for each unique value in the group by expression. Omitting group by creates a single group for the whole table.
• Aggregate functions specified in the select list calculate summary values for each group. For scalar aggregates, there is only one value for the table. Vector aggregates calculate values for the distinct groups.
• The having clause excludes groups from the results that do not meet its search conditions. Even though the having clause tests only rows, the presence or absence of a group by clause may make it appear to be operating on groups:
  • When the query includes group by, having excludes result group rows. This is why having seems to operate on groups.
  • When the query has no group by, having excludes result rows from the (single-group) table. This is why having seems to operate on rows (the results are similar to where clause results).

Secondly, a brief summary appears in the section "How the having, group by, and where clauses interact":-

How the having, group by, and where clauses interact

When you include the having, group by, and where clauses in a query, the sequence in which each clause affects the rows determines the final results:

• The where clause excludes rows that do not meet its search conditions.
• The group by clause collects the remaining rows into one group for each unique value in the group by expression.
• Aggregate functions specified in the select list calculate summary values for each group.
• The having clause excludes rows from the final results that do not meet its search conditions.

@SQLGuru's explanation is an illustration of this.

Edit...

On a related point, I was surprised by the behaviour of non-ANSI-conforming queries that utilise TSQL "extended columns". Sybase handles the extended columns (i) after the WHERE clause (ii) by creating extra joins to the original tables and (iii) the WHERE clause is not used in the join. Such queries might return more rows than expected and the HAVING clause then requires additional conditions to filter these out.

See examples b, c and d under "Transact-SQL extensions to group by and having" on the page of your originally-linked docco. I found it useful to install the pubs2 sample database from Sybase to play along with the examples.