SQL - using alias in Group By

Question

Just curious about SQL syntax  So if I have  SELECT   itemName as ItemName   substring itemName  1 1  as FirstLetter   Count itemName  FROM table1 GROUP BY itemName  FirstLetter   This would be incorrect because   GROUP BY itemName  FirstLetter    really should be   GROUP BY itemName  substring itemName  1 1    But why can t we simply use the former for convenience

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Caution that using alias in the Group By  for services that support it  such as postgres  can have unintended results  For example  if you create an alias that already exists in the inner statement  the Group By will chose the inner field name   -- Working example in postgres select col1 as col1 1  avg col3  as col2 1 from      select gender as col1  maritalstatus as col2       yearlyincome as col3 from customer  as layer 1 group by col1 1   -- Failing example in postgres select col2 as col1  avg col3  from      select gender as col1  maritalstatus as col2      yearlyincome as col3 from customer  as layer 1 group by col1

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I m not answering why it is so  but only wanted to show a way around that limitation in SQL Server by using CROSS APPLY to create the alias  You then use it in the GROUP BY clause  like so   SELECT   itemName as ItemName   FirstLetter   Count itemName  FROM table1 CROSS APPLY  SELECT substring itemName  1 1  as FirstLetter  Alias GROUP BY itemName  FirstLetter

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Back in the day I found that Rdb  the former DEC product now supported by Oracle allowed the column alias to be used in the GROUP BY   Mainstream Oracle through version 11 does not allow the column alias to be used in the GROUP BY   Not sure what Postgresql  SQL Server  MySQL  etc will or won t allow   YMMV

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Beware of using aliases when grouping the results from a view in SQLite   You will get unexpected results if the alias name is the same as the column name of any underlying tables  to the views

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SQL Server doesn t allow you to reference the alias in the GROUP BY clause because of the logical order of processing   The GROUP BY clause is processed before the SELECT clause  so the alias is not known when the GROUP BY clause is evaluated   This also explains why you can use the alias in the ORDER BY clause   Here is one source for information on the SQL Server logical processing phases

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You could always use a subquery so you can use the alias  Of course  check the performance  Possible the db server will run both the same  but never hurts to verify    SELECT ItemName  FirstLetter  COUNT ItemName  FROM       SELECT ItemName  SUBSTRING ItemName  1  1  AS FirstLetter     FROM table1       ItemNames GROUP BY ItemName  FirstLetter

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In at least Postgres  you can use the alias name in the group by clause  SELECT itemName as ItemName1  substring itemName  1 1  as FirstLetter  Count itemName  FROM table1 GROUP BY ItemName1  FirstLetter  I wouldn t recommend renaming an alias as a change in capitalization  that causes confusion

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Some DBMSs will let you use an alias instead of having to repeat the entire expression  Teradata is one such example   I avoid ordinal position notation as recommended by Bill for reasons documented in this SO question   The easy and robust alternative is to always repeat the expression in the GROUP BY clause  DRY does NOT apply to SQL

User · Answer

At least in PostgreSQL you can use the column number in the resultset in your GROUP BY clause    SELECT   itemName as ItemName   substring itemName  1 1  as FirstLetter   Count itemName  FROM table1 GROUP BY 1  2   Of course this starts to be a pain if you are doing this interactively and you edit the query to change the number or order of columns in the result  But still

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SQL is implemented as if a query was executed in the following order    FROM clause WHERE clause GROUP BY clause HAVING clause SELECT clause ORDER BY clause   For most relational database systems  this order explains which names  columns or aliases  are valid because they must have been introduced in a previous step   So in Oracle and SQL Server  you cannot use a term in the GROUP BY clause that you define in the SELECT clause because the GROUP BY is executed before the SELECT clause   There are exceptions though  MySQL and Postgres seem to have additional smartness that allows it

[sql] SQL - using alias in Group By

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