How to avoid SQL Server error on ORDER BY with duplicate columns

问题

Although this question references PHP, it is not actually PHP-specific, so I have not flagged it as such.

We have a PHP framework which supports multiple DB back-ends.

There is a generic function in our data object class, which allows you to get records from the underlying table, with a specified criteria and sort order.

It looks something like this:

function GetAll($Criteria, $OrderBy = "") {

    ...

    // Add primary key (column 1) to end of order by list,
    // so that returned order is predictable.
    if ($OrderBy != "") {
        $OrderBy .= ", ";
    }
    $OrderBy .= "1";

    ...

    // Build and run query, returning the result as an array.
}

If you specify an $OrderBy argument of StaffID on a Staff object, the resulting SQL looks something like the following:

SELECT * FROM adminStaff ORDER BY StaffID, 1;

This works fine on a MySQL back-end, and from my searching of the web it should also be fine on most other DB back-ends. However, when using SQL Server, we get the following error message:

A column has been specified more than once in the order by list.
Columns in the order by list must be unique.

This arises because SQL Server disallows the same column appearing multiple times in the ORDER BY clause. In this case StaffID is column 1 and therefore we have multiple instances of the same column.

Is there a way to disable this check in SQL Server? MySQL provides a lot of options to enable/disable strictness checks and incompatible features - does SQL Server provide anything of that nature that would allow the above query to run without errors?

If not, do you have any suggestions for how we could resolve this in our data-object layer? Bear in mind we need to maintain compatibility with existing projects which expect this behaviour, so it is not sufficient to only include the first column when $OrderBy is blank.

The situation is also slightly complicated in the fact that the field list is customisable elsewhere in the data object configuration, so we can't rely on * being used as the field list - it could contain pretty much anything that is valid in a normal SQL field list. However, if that is asking too much, a solution to the simpler case (as outlined above) would be a good start!

回答1:

In SQL Server you are able to sort either by column name or by ordinal position of the column order in the SELECT list.

In your case the column StaffID became the ordinal position 1. Hence SQL Server cannot sort the same result set based on the same column twice.

If you remove the 1 from your query, the problem will be solved.

Avoid using the ordinal position of the column for sorting.

回答2:

The basic question - is it possible to suppress this SQL Server restriction on ORDER BY column duplication - was answered by Venu: No it is not.

There are various suggestions (mostly from me) about how you could possibly code around this limitation in a generic manner. For any future readers, those answers are probably the most helpful if you are adapting an existing system. (If you are starting from scratch, just try and avoid this situation altogether.)

However, the actual solution that I came to was to add versioning to our internal API for our DBAL. The API version is now 2 but you can call setApiVersion(1) to instruct the back-end to use the old version of the API instead.

v2 is identical to v1* except it no longer automatically adds column 1 to the ORDER BY unless it is completely blank. Therefore, the SQL Server issue is resolved for new (v2) projects, whilst existing projects can be set to use the v1 API and therefore continue to work correctly (but without SQL server compatibility).

(* Actually, I've taken this opportunity to make some other breaking changes in v2, but that is not relevant to this answer.)

回答3:

I've come up with a couple of potential solutions at the framework level. All of them have performance implications which would need to be profiled, and in practice that may rule some or all of them out. However, in theory at least, these are ways that a generic solution could be implemented.

Omit the ORDER BY altogether, and do the sorting in code. Would involve parsing the provided ORDER BY string. Would be problematic if ORDER BY contained expressions, but I can't remember ever seeing that in our projects, so can probably be ignored. Probably the slowest solution.
Perform the query without the ORDER BY, limiting the results set to a single row. Use resulting column list to work out whether column 1 is already in the ORDER BY clause, and therefore whether to add it. Then run the full query. Would require parsing the provided ORDER BY string. Query caching may mean this won't add as much overhead as it appears.
Parse the field list to get the first column name and see if this appears in the ORDER BY clause. If field list contains * or table.* would require a schema lookup. May be too difficult if we need to deal with table aliases and wildcards in combination.
Parse ORDER BY string and see if it contains any primary key. If so it is already uniquely ordered and doesn't require the addition of an extra field. Would require a schema look-up.
Use a sub-select to give us a new instance of the column that we can sort on instead. Not sure whether SQL Server would still complain that this is the 'same' column, though.

回答4:

Could you just append '--' to your OrderBy parameter when working with SQL Server and just explicitly define the Order By fields where necessary?

来源：https://stackoverflow.com/questions/42766186/how-to-avoid-sql-server-error-on-order-by-with-duplicate-columns

标签

sql-server

portability