I would like to do something like
(SELECT ... FROM ...) AS my_select
WHERE id IN (SELECT MAX(id) FROM my_select GROUP BY name)
Is it possible to somehow do the "AS my_select" part (i.e. assign an alias to a SELECT statement)?
(Note: This is a theoretical question. I realize that I can do it without assign an alias to a SELECT statement, but I would like to know whether I can do it with that.)
Not sure exactly what you try to denote with that syntax, but in almost all RDBMS-es you can use a subquery in the FROM clause (sometimes called an "inline-view"):
SELECT..
FROM (
SELECT ...
FROM ...
) my_select
WHERE ...
In advanced "enterprise" RDBMS-es (like oracle, SQL Server, postgresql) you can use common table expressions which allows you to refer to a query by name and reuse it even multiple times:
-- Define the CTE expression name and column list.
WITH Sales_CTE (SalesPersonID, SalesOrderID, SalesYear)
AS
-- Define the CTE query.
(
SELECT SalesPersonID, SalesOrderID, YEAR(OrderDate) AS SalesYear
FROM Sales.SalesOrderHeader
WHERE SalesPersonID IS NOT NULL
)
-- Define the outer query referencing the CTE name.
SELECT SalesPersonID, COUNT(SalesOrderID) AS TotalSales, SalesYear
FROM Sales_CTE
GROUP BY SalesYear, SalesPersonID
ORDER BY SalesPersonID, SalesYear;
(example from http://msdn.microsoft.com/en-us/library/ms190766(v=sql.105).aspx)
You can do this using the WITH clause of the SELECT statement:
;
WITH my_select As (SELECT ... FROM ...)
SELECT * FROM foo
WHERE id IN (SELECT MAX(id) FROM my_select GROUP BY name)
That's the ANSI/ISO SQL Syntax. I know that SQL Server, Oracle and DB2 support it. Not sure about the others...
Yes, but you can select only one column in your subselect
SELECT (SELECT id FROM bla) AS my_select FROM bla2
You could store this into a temporary table.
So instead of doing the CTE/sub query you would use a temp table.
Good article on these here http://codingsight.com/introduction-to-temporary-tables-in-sql-server/
Related
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?
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.
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
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.
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.
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
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;
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.
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.)
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.
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.
I need to count all the distinct records in a table name with a single query and also without using any sub-query.
My code is
select count ( distinct *) from table_name
It gives an error:
Incorrect syntax near '*'.
I am using Microsoft SQL Server
Try this -
SELECT COUNT(*)
FROM
(SELECT DISTINCT * FROM [table_name]) A
I'm afraid that if you don't want to use a subquery, the only way to achieve that is replacing * with a concatenation of the columns in your table
select count(distinct concat(column1, column2, ..., columnN))
from table_name
To avoid undesired behaviours (like the concatenation of 1 and 31 becoming equal to the concatenation of 13 and 1) you could add a reasonable separator
select count(distinct concat(column1, '$%&£', column2, '$%&£', ..., '$%&£', columnN)
from table_name
You can use CTE.
;WITH CTE AS
(
SELECT DISTINCT * FROM TableName
)
SELECT COUNT(*)
FROM CTE
Hope this query gives you what you required.
As others mentioned, you cannot use DISTINCT with *. Also it is good practice to use a column name instead of the *, like a unique key / primary key of the table.
SELECT COUNT( DISTINCT id )
FROM table
select distinct Name , count(Name) from TableName
group by Name
having count(Name)=1
select ##rowcount
I had the same issue involving a query that had multiple joins to tables and I could not simply do count(distinct ) or count(distinct alias.).
My solution was to create a string made up of the key columns I cared about and count them.
SELECT Count(DISTINCT person.first || '~' || person.last)
from person;
If you want to use DISTINCT keyword, you need to specify column name on which bases you want to get distinct records.
Example:
SELECT count(DISTINCT Column-Name) FROM table_name
I have a t-sql query where sum function is duplicated.
How to avoid duplicating those statements?
select
Id,
sum(Value)
from
SomeTable
group by
Id
having
sum(Value) > 1000
It look like table aliasing is not supported.
I think with should work:
with tmptable (id,sumv)
as
(select
Id,
sum(Value) as sumv
from
SomeTable
group by
Id
)
select
id,
sumv
from
tmptable
where
sumv>1000
And a fiddle:
http://sqlfiddle.com/#!6/0d3f2/2
You need to remove the sum(Value) from the group by clause.
You can use GROUP BY 1, 2 to group by (in this case) the first and second columns, and thus avoid duplication in the GROUP BY clause.
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?
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.
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
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.
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.
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
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;
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.
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.)
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.
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.
This query gets the top item in each group using the ranking function.
I want to reduce the number of inner selects down to two instead of three. I tried using the rank() function in the innermost query, but couldn't get it working along with an aggregate function. Then I couldn't use a where clause on 'itemrank' without wrapping it in yet another select statement.
Any ideas?
select *
from (
select
tmp.*,
rank() over (partition by tmp.slot order by slot, itemcount desc) as itemrank
from (
select
i.name,
i.icon,
ci.slot,
count(i.itemid) as itemcount
from items i
inner join citems ci on ci.itemid = i.itemid
group by i.name, i.icon, ci.slot
) as tmp
) as popularitems
where itemrank = 1
EDIT: using sql server 2008
In Oracle and Teradata (and perhaps others too), you can use QUALIFY itemrank = 1 to get rid of the outer select. This is not part of the ANSI standard.
You can use Common Table Expressions in Oracle or in SQL Server.
Here is the syntax:
WITH expression_name [ ( column_name [,...n] ) ]
AS
( CTE_query_definition )
The list of column names is optional only if distinct names for all resulting columns are supplied in the query definition.
The statement to run the CTE is:
SELECT <column_list>
FROM expression_name;