Using 'HAVING' without 'GROUP BY' is not allowed:
SELECT *
FROM products
HAVING unitprice > avg(unitprice)
Column 'products.UnitPrice' is invalid in the HAVING clause because it is not contained in either an aggregate function or the GROUP BY clause.
But when placing the same code under 'EXISTS' - no problems:
SELECT *
FROM products p
WHERE EXISTS (SELECT 1
FROM products
HAVING p.unitprice > avg(unitprice))
Can you please explain why?
well the error is clear in first query UnitPrice is not part of aggregation nor group by
whereas in your second query you are comparing p.unitprice from table "products p" which doesn't need to be part of aggregation or group by , your second query is equivalent to :
select * from products p
where p.unitprice > (select avg(unitprice) FROM products)
which maybe this is more clear , that sql caculate avg(unitprice) then compares it with unitprice column from product.
HAVING filters after aggregation according to the SQL standard and in most databases.
Without a GROUP BY, there is still aggregation.
But in your case, you simply want a subquery and WHERE:
SELECT p.*
FROM products p
WHERE p.unitprice > (SELECT AVG(p2.unitprice) FROM products p2);
The problem comes from the columns you select :
SELECT *
and
SELECT 1
Unlike ordinary functions that are evaluated at each row, aggregate functions are computed once the whole dataset is processed, which means that in theory (at least without a GROUP BY statement), you can't select both aggregate and regular functions in a same column set (even if some DBMS still tolerate this).
It's easier to see when considering SUM(). You're not supposed to have an access to the total of a column before all rows have been returned, which prevents you to write something like SELECT price,SUM(price), for instance.
GROUP BY, now, enables you to regroup your rows according to a given criteria (actually, a bunch of columns), which makes these aggregate functions to be computed at the end of each of these groups instead of the whole dataset. Therefore, since all the column specified in GROUP BY are supposed to be the same for a given group, you're allowed to include them in your global SELECT statement.
This leads us to the actual failure cause: on first query, you select all columns. On the second one, you select none: only the constant 1, which is not part of the table itself.
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 have a query
SELECT bk_publisher, bk_price FROM books
GROUP BY bk_price, bk_publisher
and
SELECT bk_publisher ,bk_price FROM books
both are returning the same results. Means i have 12 records in my table and both queries returning the 12 records. What is the difference ? Although i am using group by, which is use with aggregate functions. But i want to know is group by making any difference here ?
SELECT bk_publisher, bk_price FROM books
GROUP BY bk_price, bk_publisher
Will result distinct pairs of (publisher, price), even if your table contains duplicated data.
SQL group by helps you group different results by some identical value (using aggregation functions on other values)
In your case it doesn't mean anything, but when you want to aggregate values based on identical field, you use group by.
For example, if you want to get the max price of a publisher:
SELECT bk_publisher, max(bk_price) FROM books
GROUP BY bk_publisher
The GROUP BY statement is used to group the result-set by one or more columns.Group by is used when you have repeating data and you want single record for each entry.
When you use GROUP BY, it will squeeze multiple rows having identical columns listed in GROUP BY as single row in output.
It also means that in general, all other columns mentioned in SELECT list must be wrapped in aggregate functions like sum(), avg(), count(), etc.
Some SQL engines like MySQL permit not using aggregates, but many people consider this a bug.
GROUP BY clause is apparently showing no effect because there is no repeating combination of bk_price, bk_publisher values.
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