Using column alias in join statement

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Using column alias in join statement

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mattt
Posting Yak Master

194 Posts

Posted - 2010-12-31 : 06:44:41

Hi,

SQL Server doesn't seem to like this statement:


select c.strId, sq.strId as intItemId from Cache.s1_visionsandbox.GlobalBlockedList c
join s1_visionsandbox_2.Item sq on sq.strId = c.strId
join s1_visionsandbox_2.Catalogue gb_cat on gb_cat.intRowId = sq.intItemId

It objects to the use of sq.intItemId in the join, saying that the column doesn't exist even though I've aliased it in the select.

Is there any way I can get this to work - renaming the column using an alias and then use it in the join?

(you might legitimately ask *why* I need to do this given that it would be simpler just to use the proper column names. The answer is that our code fits sql fragments together to created stored procedures on the fly and I can't change the fragments for fear of knock-on effects - but I can change the body of the sproc to use an alias instead)

Cheers,
Matt

dataguru1971
Master Smack Fu Yak Hacker

1464 Posts

Posted - 2010-12-31 : 07:43:37

You have to join on the original column name, not the alias. What you propose might work in Access perhaps, but not in SQL. You also prefaced the alias with the table alias prefix (sq.intItemID instead of just intItemId) and the alias doesn't exist in the table.


select c.strId, sq.strId as intItemId from Cache.s1_visionsandbox.GlobalBlockedList c
join s1_visionsandbox_2.Item sq on sq.strId = c.strId
join s1_visionsandbox_2.Catalogue gb_cat on gb_cat.intRowId = sq.strId

Poor planning on your part does not constitute an emergency on my part.

mattt
Posting Yak Master

194 Posts

Posted - 2010-12-31 : 09:10:57

Actually, it turns out you can do it in a rather roundabout fashion by using a subquery.


select sq.intItemId, sq.meh from
( select c.strId as meh, sq.strId as intItemId from Cache.s1_visionsandbox.GlobalBlockedList c
join s1_visionsandbox_2.Item sq on sq.strId = c.strId ) sq
join s1_visionsandbox_2.Catalogue gb_cat on gb_cat.intRowId = sq.intItemId

dataguru1971
Master Smack Fu Yak Hacker

1464 Posts

Posted - 2010-12-31 : 09:18:39

Ahh..i missed the bit about your requirement to use the alias due to the nature of the on the fly creation..

Poor planning on your part does not constitute an emergency on my part.

jcelko
Esteemed SQL Purist

547 Posts

Posted - 2011-01-03 : 11:55:23

Here is how a SELECT works in SQL ... at least in theory. Real products will optimize things, but the code has to produce the same results.

a) Effectively materialize the CTEs in the optional WITH clause. CTE's come into existence in the order they are declared so only backward references are alllowed. A CTE can be recursive. Think of them as VIEWs that exist only in the scope of the query. In practice, if they are used once then they are implemented as an in-line macro.

b) Start in the FROM clause and build a working table from all of the joins, unions, intersections, and whatever other table constructors are there. The <table expression> AS <correlation name> option allows you give a name to this working table which you then have to use for the rest of the containing query. Ther are UNIONB, INTERSECT and EXCEPT set construtors, LATERAL tables, table-valued funcitosn and all kinds of things happening in here.

c) Go to the WHERE clause and remove rows that do not pass criteria; that is, that do not test to TRUE (i.e. reject UNKNOWN and FALSE). The WHERE clause is applied to the working set in the FROM clause.

d) Go to the optional GROUP BY clause, partiton the original table into groups and reduce each grouping to a *single* row, replacing the original working table with the new grouped table. The rows of a grouped table must be only group characteristics: (1) a grouping column (2) a statistic about the group (i.e. aggregate functions) (3) a function or constant(4) an expression made up of only those three items. The original table no longer exists and you cannot reference anything in it (this was an error in early Sybase products).

e) Go to the optional HAVING clause and apply it against the grouped working table; if there was no GROUP BY clause, treat the entire table as one group.

f) Go to the SELECT clause and construct the expressions in the list. This means that the scalar subqueries, function calls and expressions in the SELECT are done after all the other clauses are done. The AS operator can also give names to expressions in the SELECT list. These new names come into existence all at once, but after the WHERE clause, GROUP BY clause and HAVING clause have been executed; you cannot use them in the SELECT list or the WHERE clause for that reason.

If there is a SELECT DISTINCT, then redundant duplicate rows are removed. For purposes of defining a duplicate row, NULLs are treated as matching (just like in the GROUP BY).

g) Nested query expressions follow the usual scoping rules you would expect from a block structured language like C, Pascal, Algol, etc. Namely, the innermost queries can reference columns and tables in the queries in which they are contained.

h) The ORDER BY clause is part of a cursor, not a query. The result set is passed to the cursor, which can only see the names in the SELECT clause list, and the sorting is done there. The ORDER BY clause cannot have expression in it, or references to other columns because the result set has been converted into a sequential file structure and that is what is being sorted.

As you can see, things happen "all at once" in SQL, not "from left to right" as they would in a sequential file/procedural language model. In those languages, these two statements produce different results:
READ (a, b, c) FROM File_X;
READ (c, a, b) FROM File_X;

while these two statements return the same data:

SELECT a, b, c FROM Table_X;
SELECT c, a, b FROM Table_X;

Think about what a confused mess this statement is in the SQL model.

SELECT f(c2) AS c1, f(c1) AS c2 FROM Foobar;

That is why such nonsense is illegal syntax.

--CELKO--
Books in Celko Series for Morgan-Kaufmann Publishing
Analytics and OLAP in SQL
Data and Databases: Concepts in Practice
Data, Measurements and Standards in SQL
SQL for Smarties
SQL Programming Style
SQL Puzzles and Answers
Thinking in Sets
Trees and Hierarchies in SQL

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