I never notice below before:
SELECT A,*
FROM Table
ORDER BY A
It gives me the Ambiguous column name error. The * of course contains A, but will sql server take the same column A in above query as two different columns? Is this the reason behind that?
it is because the output has the same name if you did
SELECT A AS A_STANDS_ALONE,*
FROM Table
ORDER BY A
It will work
If you are looking for yes/no answer, then 'YES', you are right.
SQL Selects A column twice, one by explicit and one by the implicit * call.
SQL doesn't understand which one you want to sort by.
The first comment to your post shows that you can use an alias for the column.
Related
So in Hue I've entered a simple query(has to be as simple as possible, as others will run it too) to just get a limit of 20 records. The query is:
Select * from tablename Limit 20
The problem is that the query returns column names in this format: tablename.columnname
I need JUST the column name to be returned, NOT the table name referenced at all. How is this achieved without going into a large "from" statement spelling out all of the columns(only other way I currently know)?
Thanks in advance!
Not the best but you could do a right click on the '*' of SELECT * and then expand all the column names:
The following code:
SELECT JaguarStartupTime, CPU, AmountOfRam, UpdatedOn, *
FROM dbo.MachineConfiguration
WHERE ServerName = 'WashingtonDC01'
AND UpdatedOn > '11/21/2012'
ORDER BY JaguarStartupTime DESC
results in an error: Ambiguous column name 'JaguarStartupTime'.
However, removing the ORDER BY makes it work. In addition, prefixing the ORDER BY clause with the table, like below, makes it work too:
SELECT JaguarStartupTime, CPU, AmountOfRam, UpdatedOn, *
FROM dbo.MachineConfiguration
WHERE ServerName = 'WashingtonDC01'
AND UpdatedOn > '11/21/2012'
ORDER BY dbo.MachineConfiguration.JaguarStartupTime DESC
This never made sense to me. Can someone explain?
Because the query includes * (all columns) in the SELECT clause, which also includes the column JaguarStartupTime.
In Layman's terms:
So in the retrieval of the results, the column shows up twice, and then when the server tries to apply the sort order, it's not sure which column you are referring to.
Personally, I'd change the query to eliminate the use of * in the query, even if it means listing a long list of column names. It's best practice, and it will avoid this issue.
As for why it works when prefixed, I found this in the MSDN documentation:
In SQL Server, qualified column names and aliases are resolved to
columns listed in the FROM clause. If order_by_expression is not
qualified, the value must be unique among all columns listed in the
SELECT statement.
This is simply part of the DB engine's underlying mechanism for translating SQL statements into the execution plan.
I agree that in the case in your question ORDER BY JaguarStartupTime is not really ambiguous as both projected columns in fact refer to the same underlying column.
In the general case however there is no guarantee that this will be true. As discussed in the response to this connect item. For example in the following query it is clearly ambiguous which one would be used.
SELECT JaguarStartupTime,
CPU AS JaguarStartupTime
FROM dbo.MachineConfiguration
ORDER BY JaguarStartupTime DESC
For syntax validation purposes SQL Server does not do any analysis of column names to determine whether they are really ambiguous or not. When ordering by some_name that is present in the list of projected column names the some_name must be unique. This is as per the ANSI spec
If a <sort specification> contains a <column name>, then T
shall contain exactly one column with that <column name> and
the <sort specification> identifies that column.
Qualifying the column name with the table name ensures to SQL Server that you are ordering by a source column name rather than a projected column name so it allows it.
SELECT name, name
FROM master..spt_values v1
ORDER BY v1.name
Additionally the following query is allowed.
select name, name
FROM master..spt_values v1
ORDER BY name + ''
The reason why this is not ambiguous is because column aliases in an ORDER BY clause can only be used by themselves rather than in an expression so in the above query name can not be interpreted as an alias and must be interpreted as ordering by v1.name.
Does the result of SELECT MAX([ColumnName]) have a column name? I know the result is a view of only one record and I'd thought this view has the original column name as the result column name?
It does not but you can give it one
SELECT MAX([ColumnName]) AS MaxName
If you think about it you will see why your assumption is wrong. What if the code was like this:
SELECT MAX([ColumnName]), MIN([ColumnName])
Which of the two will take the name of the column?
By default no, there is no particular column name. If you specify an alias it will have.
SELECT MAX([ColumnName]) as [max_value_column_name]
I know you cannot use a alias column in the where clause for T-SQL; however, has Microsoft provided some kind of workaround for this?
Related Questions:
Unknown Column In Where Clause
Can you use an alias in the WHERE clause in mysql?
“Invalid column name” error on SQL statement from OpenQuery results
One workaround would be to use a derived table.
For example:
select *
from
(
select a + b as aliased_column
from table
) dt
where dt.aliased_column = something.
I hope this helps.
Depending on what you are aliasing, you could turn it into a user defined function and reference that in both places. Otherwise your copying the aliased code in several places, which tends to become very ugly and means updating 3+ spots if you are also ordering on that column.
I'm trying to create a sql query, but there is this error:
Ambiguous column name 'description'.
Its because this column occurs in both tables.
if I remove the description from the query, it works.
I tried to rename the description-field "AS description_pointer", but the error still occurs.
SELECT TOP 1000 [activityid]
,[activitytypecodename]
,[subject]
,[regardingobjectid]
,[contactid]
,[new_crmid]
,[description] AS description_pointer
FROM [crmtestext_MSCRM].[dbo].[FilteredActivityPointer] as I
Left JOIN [crmtestext_MSCRM].[dbo].[FilteredContact]
ON I.[regardingobjectid] = [crmtestext_MSCRM].[dbo].[FilteredContact].[contactid]
WHERE new_crmid not like '%Null%' AND activitytypecodename like '%E-mail%'
Both tables coming into play in the query have a column named description. You RDBMS cannot guess which column table you actually want.
You need to prefix the column name with the table name (or table alias) to disambiguate it.
Bottom line, it is a good practice to always prefix column names with table names or aliases as soon as several tables come into play in a query. This avoids the issue that you are seeing here and make the queries easier to understand for the poor souls that have no knowledge of the underlying schema.
Here is an updated version of your query with table aliases and column prefixes. Obviously you need to review each column to put the correct alias:
SELECT TOP 1000
i.[activityid]
,i.[activitytypecodename]
,i.[subject]
,c.[regardingobjectid]
,c.[contactid]
,c.[new_crmid]
,c.[description] AS description_pointer
FROM [crmtestext_MSCRM].[dbo].[FilteredActivityPointer] as i
Left JOIN [crmtestext_MSCRM].[dbo].[FilteredContact] as c
ON i.[regardingobjectid] = c.[contactid]
WHERE i.new_crmid not like '%Null%' AND i.activitytypecodename like '%E-mail%'