I keep getting an error saying this is not a group by expression when i run this statement in oracle XE.
SELECT PROJECTID, HOURSWORKED FROM ASSIGNMENT GROUP BY PROJECTID HAVING HOURSWORKED > 20;
GROUP BY is used for an aggregating expression, such as MAX() or SUM().
You can either write
SELECT PROJECTID, HOURSWORKED
FROM ASSIGNMENT
WHERE HOURSWORKED > 20;
or
SELECT PROJECTID, SUM(HOURSWORKED)
FROM ASSIGNMENT
GROUP BY PROJECTID
HAVING SUM(HOURSWORKED) > 20;
(or some other aggregate function, eg: MAX, MIN) depending on your intent.
WHERE is used to filter individual records
HAVING is used to filter the grouped aggregate values.
You have to have an aggregate function on hoursworked - e.g. MAX(HOURSWORKED), AVERAGE(HOURSWORKED) etc. Or you also have to add it to the group by, though that would seem strange..
One other suggestion for asking SQL questions on stackoverflow - create a minimal test case using SQL Fiddle. E.g. http://sqlfiddle.com/#!17/fea91/5 for your example. Unless it's something very Oracle specific, Postgres is usually close enough for ordinary SQL questions. Try changing the WHERE to a HAVING, to see the difference between the two...
I'm using oracle sql developer and I can't get this query to function. It's telling me its not a single group function. Please help.
SELECT LGBRAND.BRAND_NAME, LGPRODUCT.PROD_DESCRIPT,
COUNT (LGPRODUCT.PROD_DESCRIPT) AS "NUMPRODUCTS"
FROM LGBRAND, LGPRODUCT
ORDER BY LGBRAND.BRAND_NAME;
What I'm trying to accomplish is to get the total different products grouped by each brand name.
when using aggregate functions you need to use group by clause
All aggregate functions like avg, count,sum needs to be used along with a group by function. If you dont use a group by clause, you are performing the function on all the rows of the table.
SELECT LGBRAND.BRAND_NAME,
LGPRODUCT.PROD_DESCRIPT,
COUNT (LGPRODUCT.PROD_DESCRIPT) AS "NUMPRODUCTS"
FROM LGBRAND, LGPRODUCT,
GROUP BY LGBRAND.BRAND;
You need to use GROUP BY clause.
I ran a query against Northwind database Products Table like below
select * from Northwind.dbo.Products GROUP BY CategoryID and i was hit with a error. I am sure you will also be hit by same error. So what is the correct statement that i need to execute to group all products with respect to their category id's.
edit: this like really helped understand a lot
http://weblogs.sqlteam.com/jeffs/archive/2007/07/20/but-why-must-that-column-be-contained-in-an-aggregate.aspx
You need to use an Aggregate function and then group by any non-aggregated columns.
I recommend reading up on GROUP BY.
If you're using GROUP BY in a query, all items in your SELECT statement must either be contained as part of an aggregate function, e.g. Sum() or Count(), else they will also need to be included in the GROUP BY clause.
Because you are using SELECT *, this is equivalent to listing ALL columns in your SELECT.
Therefore, either list them all in the GROUP BY too, use aggregating functions for the rest where possible, or only select the CategoryID.
i tried using this query:
"SELECT * FROM guests WHERE event_id=".$id." GROUP BY member_id;"
and I'm getting this error:
ERROR: column "guests.id" must appear in the GROUP BY clause or be used in an aggregate function
can anyone explain how i can work around this?
You can't Group By without letting the Select know what to take, and how to group.
Try
SELECT guests.member_id FROM guests WHERE event_id=".$id." GROUP BY member_id;
IF you need to get more info from this table about the guests, you'll need to add it to the Group By.
Plus, it seems like your select should actually be
SELECT guests.id FROM guests WHERE event_id=".$id." GROUP BY id;
Each of the columns used in a group by query needs to be specifically called out (ie, don't do SELECT * FROM ...), as you need to use them in some sort of aggregate function (min/max/sum/avg/count/etc) or be part of the group by clause.
For example:
SELECT instrument, detector, min(date_obs), max(date_obs)
FROM observations
WHERE observatory='SOHO'
GROUP BY instrument, detector;
What is the difference between HAVING and WHERE in an SQL SELECT statement?
EDIT: I have marked Steven's answer as the correct one as it contained the key bit of information on the link:
When GROUP BY is not used, HAVING behaves like a WHERE clause
The situation I had seen the WHERE in did not have GROUP BY and is where my confusion started. Of course, until you know this you can't specify it in the question.
HAVING: is used to check conditions after the aggregation takes place.
WHERE: is used to check conditions before the aggregation takes place.
This code:
select City, CNT=Count(1)
From Address
Where State = 'MA'
Group By City
Gives you a table of all cities in MA and the number of addresses in each city.
This code:
select City, CNT=Count(1)
From Address
Where State = 'MA'
Group By City
Having Count(1)>5
Gives you a table of cities in MA with more than 5 addresses and the number of addresses in each city.
HAVING specifies a search condition for a
group or an aggregate function used in SELECT statement.
Source
Number one difference for me: if HAVING was removed from the SQL language then life would go on more or less as before. Certainly, a minority queries would need to be rewritten using a derived table, CTE, etc but they would arguably be easier to understand and maintain as a result. Maybe vendors' optimizer code would need to be rewritten to account for this, again an opportunity for improvement within the industry.
Now consider for a moment removing WHERE from the language. This time the majority of queries in existence would need to be rewritten without an obvious alternative construct. Coders would have to get creative e.g. inner join to a table known to contain exactly one row (e.g. DUAL in Oracle) using the ON clause to simulate the prior WHERE clause. Such constructions would be contrived; it would be obvious there was something was missing from the language and the situation would be worse as a result.
TL;DR we could lose HAVING tomorrow and things would be no worse, possibly better, but the same cannot be said of WHERE.
From the answers here, it seems that many folk don't realize that a HAVING clause may be used without a GROUP BY clause. In this case, the HAVING clause is applied to the entire table expression and requires that only constants appear in the SELECT clause. Typically the HAVING clause will involve aggregates.
This is more useful than it sounds. For example, consider this query to test whether the name column is unique for all values in T:
SELECT 1 AS result
FROM T
HAVING COUNT( DISTINCT name ) = COUNT( name );
There are only two possible results: if the HAVING clause is true then the result with be a single row containing the value 1, otherwise the result will be the empty set.
The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions.
Check out this w3schools link for more information
Syntax:
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value
A query such as this:
SELECT column_name, COUNT( column_name ) AS column_name_tally
FROM table_name
WHERE column_name < 3
GROUP
BY column_name
HAVING COUNT( column_name ) >= 3;
...may be rewritten using a derived table (and omitting the HAVING) like this:
SELECT column_name, column_name_tally
FROM (
SELECT column_name, COUNT(column_name) AS column_name_tally
FROM table_name
WHERE column_name < 3
GROUP
BY column_name
) pointless_range_variable_required_here
WHERE column_name_tally >= 3;
The difference between the two is in the relationship to the GROUP BY clause:
WHERE comes before GROUP BY; SQL evaluates the WHERE clause before it groups records.
HAVING comes after GROUP BY; SQL evaluates HAVING after it groups records.
References
SQLite SELECT Statement Syntax/Railroad Diagram
Informix SELECT Statement Syntax/Railroad Diagram
HAVING is used when you are using an aggregate such as GROUP BY.
SELECT edc_country, COUNT(*)
FROM Ed_Centers
GROUP BY edc_country
HAVING COUNT(*) > 1
ORDER BY edc_country;
WHERE is applied as a limitation on the set returned by SQL; it uses SQL's built-in set oeprations and indexes and therefore is the fastest way to filter result sets. Always use WHERE whenever possible.
HAVING is necessary for some aggregate filters. It filters the query AFTER sql has retrieved, assembled, and sorted the results. Therefore, it is much slower than WHERE and should be avoided except in those situations that require it.
SQL Server will let you get away with using HAVING even when WHERE would be much faster. Don't do it.
WHERE clause does not work for aggregate functions
means : you should not use like this
bonus : table name
SELECT name
FROM bonus
GROUP BY name
WHERE sum(salary) > 200
HERE Instead of using WHERE clause you have to use HAVING..
without using GROUP BY clause, HAVING clause just works as WHERE clause
SELECT name
FROM bonus
GROUP BY name
HAVING sum(salary) > 200
Difference b/w WHERE and HAVING clause:
The main difference between WHERE and HAVING clause is, WHERE is used for row operations and HAVING is used for column operations.
Why we need HAVING clause?
As we know, aggregate functions can only be performed on columns, so we can not use aggregate functions in WHERE clause. Therefore, we use aggregate functions in HAVING clause.
One way to think of it is that the having clause is an additional filter to the where clause.
A WHERE clause is used filters records from a result. The filter occurs before any groupings are made. A HAVING clause is used to filter values from a group
In an Aggregate query, (Any query Where an aggregate function is used) Predicates in a where clause are evaluated before the aggregated intermediate result set is generated,
Predicates in a Having clause are applied to the aggregate result set AFTER it has been generated. That's why predicate conditions on aggregate values must be placed in Having clause, not in the Where clause, and why you can use aliases defined in the Select clause in a Having Clause, but not in a Where Clause.
I had a problem and found out another difference between WHERE and HAVING. It does not act the same way on indexed columns.
WHERE my_indexed_row = 123 will show rows and automatically perform a "ORDER ASC" on other indexed rows.
HAVING my_indexed_row = 123 shows everything from the oldest "inserted" row to the newest one, no ordering.
When GROUP BY is not used, the WHERE and HAVING clauses are essentially equivalent.
However, when GROUP BY is used:
The WHERE clause is used to filter records from a result. The
filtering occurs before any groupings are made.
The HAVING clause is used to filter values from a group (i.e., to
check conditions after aggregation into groups has been performed).
Resource from Here
From here.
the SQL standard requires that HAVING
must reference only columns in the
GROUP BY clause or columns used in
aggregate functions
as opposed to the WHERE clause which is applied to database rows
While working on a project, this was also my question. As stated above, the HAVING checks the condition on the query result already found. But WHERE is for checking condition while query runs.
Let me give an example to illustrate this. Suppose you have a database table like this.
usertable{ int userid, date datefield, int dailyincome }
Suppose, the following rows are in table:
1, 2011-05-20, 100
1, 2011-05-21, 50
1, 2011-05-30, 10
2, 2011-05-30, 10
2, 2011-05-20, 20
Now, we want to get the userids and sum(dailyincome) whose sum(dailyincome)>100
If we write:
SELECT userid, sum(dailyincome) FROM usertable WHERE
sum(dailyincome)>100 GROUP BY userid
This will be an error. The correct query would be:
SELECT userid, sum(dailyincome) FROM usertable GROUP BY userid HAVING
sum(dailyincome)>100
WHERE clause is used for comparing values in the base table, whereas the HAVING clause can be used for filtering the results of aggregate functions in the result set of the query
Click here!
When GROUP BY is not used, the WHERE and HAVING clauses are essentially equivalent.
However, when GROUP BY is used:
The WHERE clause is used to filter records from a result. The
filtering occurs before any groupings are made.
The HAVING clause is
used to filter values from a group (i.e., to check conditions after
aggregation into groups has been performed).
I use HAVING for constraining a query based on the results of an aggregate function. E.G. select * in blahblahblah group by SOMETHING having count(SOMETHING)>0