Sybase select variable logic

问题

Ok, I have a question relating to an issue I've previously had. I know how to fix it, but we are having problems trying to reproduce the error.

We have a series of procedures that create records based on other records. The records are linked to the primary record by way of a link_id. In a procedure that grabs this link_id, the query is

select @p_link_id = id --of the parent
from  table
where thingy_id = (blah)

Now, there are multiple rows in the table for the activity. Some can be cancelled. The code I have doesn't disinclude cancelled rows in the select statement, so if there are previously cancelled rows, those ids will appear in the select. There is always going to be one 'open' record that is selected if I disinclude cancelled rows. (append where status != 'C')

This solves this issue. However, I need to be able to reproduce the issue in our development environment.

I've gone through a process where I've entered a whole heap of data, opening, cancelling, etc to try and get this select statement to return an invalid id. However, whenever I run the select, the ids are in order (sequence generated), but in the case where this error occured, the select statement returned what seems to be the first value into the variable.

For example.

ID   Status
1    Cancelled
2    Cancelled
3    Cancelled
4    Open

Given the above, if I do a select for the ID I want, I want to get '4'. In the error, the result is 1. However, even if I enter in 10 cancelled records, I still get the last one in the select.

In oracle, I know that if you select into a variable and more than one record is returned, you get an error (I think). Sybase apparently can assign multiple values into a variable without erroring.

I'm thinking that there's either something to do with how the data is selected from the table, where the id's without a sort order don't return in ascending order, or there's a dboption where a select into a variable will save the first or last value queried.

Edit: it looks like we can reproduce this error by rolling back stored procedure changes. However, the procs don't go anywhere near this link_id column. Is it possible that changes to the database architecture could break an index or something?

回答1:

If more than one row is returned, the value that is stored will be the last value in the list, according to this.

If you haven't specified an order for retrieval via ORDER BY, then the order returned will be at the convenience of the database engine. It may very well vary by the database instance. It may be in the order created, or even appear "random" because of where the data is placed within the database block structure.

The moral of the story:

1. Always make singleton SELECTs return a single row
2. When #1 can't be done, use an ORDER BY to make sure the one you care about comes last

来源：https://stackoverflow.com/questions/1156837/sybase-select-variable-logic