This question already has an answer here:
Why does "select count(*)" from nothing return 1
(1 answer)
Closed 7 years ago.
for a quick check I used a query
select COUNT(*) LargeTable
and was surprized to see
LargeTable
-----------
1
seconds later I realized my mistake, made it
select COUNT(*) from LargeTable
and got expected result
(No column name)
-----------
1.000.000+
but now I don't understand why COUNT(*) returned 1
it happens if I do select COUNT(*) or declare #x int = COUNT(*); select #x
another case
declare #EmptyTable table ( Value int )
select COUNT(*) from #EmptyTable
returns
(No column name)
-----------
0
I did't find explanation in SQL standard (http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt, online source is given here https://stackoverflow.com/a/8949764/1506454)
why COUNT(*) returns 1?
In SQL Server a SELECT without a FROM clause works as though it operates against a single row table.
This is not standard SQL. Other RDBMSs provide a utility DUAL table with a single row.
So this would be treated effectively the same as
SELECT COUNT(*) AS LargeTable
FROM DUAL
A related Connect Item discussing
SELECT 'test'
WHERE EXISTS (SELECT *)
is https://connect.microsoft.com/SQLServer/feedback/details/671475/select-test-where-exists-select
Because without the FROM clause DBMS cannot know [LargeTable] is a table. You tricked it in guessing it's a COLUMN NAME alias
You can try it and see
select count(*) 'eklmnjdklfgm'
select count(*) eklmnjdklfgm
select count(*) [eklmnjdklfgm]
select count(*)
The first 3 examples returns eklmnjdklfgm as column name
Count(*) returned 1 because your sentence is not of SQL.
1) In the first sentences, you accounts a table empty, with an only row, since you don't put to which table want to access.
2) In the second case:
declare #EmptyTable table ( Value int )
select COUNT(*) from #EmptyTable
returns
(No column name)
-----------
0
You put the variable, but don't determine to which table implement him. For this, do a count and go out 0.
Related
Apologies if this question had been asked before (it probably did). I never used SQL before and the answers I've got only got me more confused.
I need to find out if an ID exists on different tables and get the total number from all tables.
Here is my query:
select * from public.ui1, public.ui2, public.ui3 where id = '123'
So if id 123 doesn't exist in ui1 and ui2 but does exist in ui3, I'd still like to get it. (I would obviously like to get it if it exists in the other tables)
I am currently getting an ambiguous error message as id exists in all tables but I am not sure how to construct this query in the appropriate manner. I tried join but failed miserably. Any help on how to reconstruct it and a stupid proof explanation would be highly appreciated!
EDIT: What I would finally like to find out is if id = 123 exists in any of the tables.
It's a bit unclear what the result is you expect. If you want the count then you can use a UNION ALL
select 'ui1' as source_table,
count(*) as num_rows
from public.ui1
where id = 123
union all
select 'ui2',
count(*)
from public.ui2
where id = 123
union all
select 'ui3',
count(*)
from public.ui3
where id = 123
If you only want to know if the id exists in at least one of the tables (so a true/false) result you can use:
select exists (select id from ui1 where id = 123
union all
select id from ui2 where id = 123
union all
select id from ui3 where id = 123)
What I would finally like to find out is if id = 123 exists in any of the tables.
The best way to do this is probably just using exists:
select v.id,
(exists (select 1 from public.ui1 t where t.id = v.id) or
exists (select 1 from public.ui2 t where t.id = v.id) or
exists (select 1 from public.ui3 t where t.id = v.id)
) as exists_flag
from (values (123)) v(id);
As written, this returns one row per id defined in values(), along with a flag of whether or not the id exists -- the question you are asking.
This can easily be tweaked if you want additional information, such as which tables the id exists in, or the number of times each appears.
I am trying to update a column called Number_Of_Marks in our Results table using the results we get from our SELECT statement. Our select statement is used to count the numbers of marks per module in our results table. The SELECT statement works and the output is correct, which is
ResultID ModuleID cnt
-------------------------
111 ART3452 2
114 ART3452 2
115 CSC3039 3
112 CSC3039 3
113 CSC3039 3
The table in use is:
Results: ResultID, ModuleID, Number_Of_Marks
We need the results of cnt to be updated into our Number_Of_Marks column. This is our code below...
DECLARE #cnt INT
SELECT #cnt
SELECT C.cnt
FROM Results S
INNER JOIN (SELECT ModuleID, count(ModuleID) as cnt
FROM Results
GROUP BY ModuleID) C ON S.ModuleID = C.ModuleID
UPDATE Results
SET [Number_Of_Marks] = (#cnt)
You can do this in SQL Server using the update/join syntax:
UPDATE s
SET [Number_Of_Marks] = c.cnt
FROM Results S INNER JOIN
(SELECT ModuleID, count(ModuleID) as cnt
FROM Results
GROUP BY ModuleID
) C
ON S.ModuleID = C.ModuleID;
I assume that you want the count from the subquery, not from the uninitialized variable.
EDIT:
In general, when you change the question it is better to ask another question. Sometimes, though, the changes are really small. The revised query looks something like:
UPDATE s
SET [Number_Of_Marks] = c.cnt,
Marks = avgmarks
FROM Results S INNER JOIN
(SELECT ModuleID, count(ModuleID) as cnt, avg(marks * 1.0) as avgmarks
FROM Results
GROUP BY ModuleID
) C
ON S.ModuleID = C.ModuleID;
Note that I multiplied the marks by 1.0. This is a quick-and-dirty way to convert an integer to a numeric value. SQL Server takes averages on integers and produces an integer. Usually you want some sort of decimal or floating value.
Given a table in SQL-Server like:
Id INTEGER
A VARCHAR(50)
B VARCHAR(50)
-- Some other columns
with no index on A or B, I wish to find rows where a unique combination of A and B occurs more than once.
I'm using the query
SELECT A+B, Count(A+B) FROM MyTable
GROUP BY A+B
HAVING COUNT(A+B) > 1
First Question
Is there a more time-efficient way to do this? (I cannot add indices to the database)
Second Question
When I attempt to gain some formatting of the output by including a , in the concatenation:
SELECT A+','+B, Count(A+','+B) FROM MyTable
GROUP BY A+','+B
HAVING COUNT(A+','+B) > 1
The query fails with the error
Column 'MyDB.dbo.MyTable.A' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause
with a similar error for Column B.
How can I format the output to separate the two columns?
It would seem more natural to me to write:
SELECT A, B, Count(*) FROM MyTable
GROUP BY A, B
HAVING COUNT(*) > 1
And it's the most efficient way of doing it (and so is the query in the question).
Similarly to the above query, you can rewrite your second query:
SELECT A + ',' + B, Count(*) FROM MyTable
GROUP BY A, B
HAVING COUNT(*) > 1
I would like to retrieve all rows matching a set of conditions on the same column. But I would like the rows only if ALL the conditions are good, and no row if only one condition fails.
For example, taking this table:
|id|name|
---------
|1 |toto|
|2 |tata|
I would like to be able to request if "tata" && "toto" are in this table. But when asking if "tata" and "tuto" are in, I would like an empty response if one of argument is in not in the table, for example asking if "toto" && "tutu" are included in the table.
How can I do that ?
Currently, I'am doing one query per argument, which is not very efficient. I tried several solutions including a subselect or a group+having, but no one is working like I want.
thanks for your support !
cheers
This isn't the most efficient way, but this query would work.
SELECT * FROM table_name
WHERE (name = 'toto' OR name = 'tata')
AND ( SELECT COUNT(*) FROM table_name WHERE name = 'toto') > 0
AND ( SELECT COUNT(*) FROM table_name WHERE name = 'tata') > 0
This is a little vague. If the names are unique, you could count the matching rows that match a where clause:
where name='toto' or name='tata'
If the count is 2, then you know both matched. If name is not unique you could potentially select the first ID (select top 1 id ...) that matches each in a union and count those with an outer select.
Even if you had an arbitrary number of names to match, you could create a stored procedure or code in whatever top-level language you are using to build the select statement.
SELECT 1 AS found FROM hehe
WHERE 1 IN (SELECT 1 FROM hehe WHERE name='tata')
AND 1 IN (SELECT 1 FROM hehe WHERE name='toto')
If name is unique you can simplify to:
SELECT *
FROM tbl
WHERE name IN ('toto', 'tata')
AND (SELECT count(*) FROM tbl WHERE name IN ('toto', 'tata')) > 1;
If it isn't:
SELECT *
FROM tbl
WHERE name IN ('toto', 'tata')
AND EXISTS (SELECT * FROM tbl WHERE name = 'toto')
AND EXISTS (SELECT * FROM tbl WHERE name = 'tata');
Or, in PostgreSQL, MySQL and possibly others:
SELECT *
FROM tbl
WHERE name IN ('toto', 'tata')
AND (SELECT count(DISTINCT name) FROM tbl WHERE name IN ('toto', 'tata')) > 1;
While reading some SQL Tuning-related documentation, I found this:
SELECT COUNT(*) :
Counts the number of rows.
Often is improperly used to verify the existence of a record.
Is SELECT COUNT(*) really that bad?
What's the proper way to verify the existence of a record?
It's better to use either of the following:
-- Method 1.
SELECT 1
FROM table_name
WHERE unique_key = value;
-- Method 2.
SELECT COUNT(1)
FROM table_name
WHERE unique_key = value;
The first alternative should give you no result or one result, the second count should be zero or one.
How old is the documentation you're using? Although you've read good advice, most query optimizers in recent RDBMS's optimize SELECT COUNT(*) anyway, so while there is a difference in theory (and older databases), you shouldn't notice any difference in practice.
I would prefer not use Count function at all:
IF [NOT] EXISTS ( SELECT 1 FROM MyTable WHERE ... )
<do smth>
For example if you want to check if user exists before inserting it into the database the query can look like this:
IF NOT EXISTS ( SELECT 1 FROM Users WHERE FirstName = 'John' AND LastName = 'Smith' )
BEGIN
INSERT INTO Users (FirstName, LastName) VALUES ('John', 'Smith')
END
You can use:
SELECT 1 FROM MyTable WHERE <MyCondition>
If there is no record matching the condition, the resulted recordset is empty.
You can use:
SELECT 1 FROM MyTable WHERE... LIMIT 1
Use select 1 to prevent the checking of unnecessary fields.
Use LIMIT 1 to prevent the checking of unnecessary rows.
SELECT COUNT(1) FROM MyTable WHERE ...
will loop thru all the records. This is the reason it is bad to use for record existence.
I would use
SELECT TOP 1 * FROM MyTable WHERE ...
After finding 1 record, it will terminate the loop.
The other answers are quite good, but it would also be useful to add LIMIT 1 (or the equivalent, to prevent the checking of unnecessary rows.
You can use:
SELECT COUNT(1) FROM MyTable WHERE ...
or
WHERE [NOT] EXISTS
( SELECT 1 FROM MyTable WHERE ... )
This will be more efficient than SELECT * since you're simply selecting the value 1 for each row, rather than all the fields.
There's also a subtle difference between COUNT(*) and COUNT(column name):
COUNT(*) will count all rows, including nulls
COUNT(column name) will only count non null occurrences of column name
Other option:
SELECT CASE
WHEN EXISTS (
SELECT 1
FROM [MyTable] AS [MyRecord])
THEN CAST(1 AS BIT) ELSE CAST(0 AS BIT)
END
I'm using this way:
IF (EXISTS (SELECT TOP 1 FROM Users WHERE FirstName = 'John'), 1, 0) AS DoesJohnExist