I have the following two tables:
1. Lecturers (LectID, Fname, Lname, degree).
2. Lecturers_Specialization (LectID, Expertise).
I want to find the lecturer with the most Specialization.
When I try this, it is not working:
SELECT
L.LectID,
Fname,
Lname
FROM Lecturers L,
Lecturers_Specialization S
WHERE L.LectID = S.LectID
AND COUNT(S.Expertise) >= ALL (SELECT
COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);
But when I try this, it works:
SELECT
L.LectID,
Fname,
Lname
FROM Lecturers L,
Lecturers_Specialization S
WHERE L.LectID = S.LectID
GROUP BY L.LectID,
Fname,
Lname
HAVING COUNT(S.Expertise) >= ALL (SELECT
COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);
What is the reason? Thanks.
WHERE clause introduces a condition on individual rows; HAVING clause introduces a condition on aggregations, i.e. results of selection where a single result, such as count, average, min, max, or sum, has been produced from multiple rows. Your query calls for a second kind of condition (i.e. a condition on an aggregation) hence HAVING works correctly.
As a rule of thumb, use WHERE before GROUP BY and HAVING after GROUP BY. It is a rather primitive rule, but it is useful in more than 90% of the cases.
While you're at it, you may want to re-write your query using ANSI version of the join:
SELECT L.LectID, Fname, Lname
FROM Lecturers L
JOIN Lecturers_Specialization S ON L.LectID=S.LectID
GROUP BY L.LectID, Fname, Lname
HAVING COUNT(S.Expertise)>=ALL
(SELECT COUNT(Expertise) FROM Lecturers_Specialization GROUP BY LectID)
This would eliminate WHERE that was used as a theta join condition.
First we should know the order of execution of Clauses i.e
FROM > WHERE > GROUP BY > HAVING > DISTINCT > SELECT > ORDER BY.
Since WHERE Clause gets executed before GROUP BY Clause the records cannot be filtered by applying WHERE to a GROUP BY applied records.
"HAVING is same as the WHERE clause but is applied on grouped records".
first the WHERE clause fetches the records based on the condition then the GROUP BY clause groups them accordingly and then the HAVING clause fetches the group records based on the having condition.
HAVING operates on aggregates. Since COUNT is an aggregate function, you can't use it in a WHERE clause.
Here's some reading from MSDN on aggregate functions.
WHERE clause can be used with SELECT, INSERT, and UPDATE statements, whereas HAVING can be used only with SELECT statement.
WHERE filters rows before aggregation (GROUP BY), whereas HAVING filter groups after aggregations are performed.
Aggregate function cannot be used in WHERE clause unless it is in a subquery contained in HAVING clause, whereas aggregate functions can be used in HAVING clause.
Source
Didn't see an example of both in one query. So this example might help.
/**
INTERNATIONAL_ORDERS - table of orders by company by location by day
companyId, country, city, total, date
**/
SELECT country, city, sum(total) totalCityOrders
FROM INTERNATIONAL_ORDERS with (nolock)
WHERE companyId = 884501253109
GROUP BY country, city
HAVING country = 'MX'
ORDER BY sum(total) DESC
This filters the table first by the companyId, then groups it (by country and city) and additionally filters it down to just city aggregations of Mexico. The companyId was not needed in the aggregation but we were able to use WHERE to filter out just the rows we wanted before using GROUP BY.
You can not use where clause with aggregate functions because where fetch records on the basis of condition, it goes into table record by record and then fetch record on the basis of condition we have give. So that time we can not where clause. While having clause works on the resultSet which we finally get after running a query.
Example query:
select empName, sum(Bonus)
from employees
order by empName
having sum(Bonus) > 5000;
This will store the resultSet in a temporary memory, then having clause will perform its work. So we can easily use aggregate functions here.
1.
We can use aggregate function with HAVING clause not by WHERE clause e.g. min,max,avg.
2.
WHERE clause eliminates the record tuple by tuple
HAVING clause eliminates entire group from the collection of group
Mostly HAVING is used when you have groups of data and WHERE is used when you have data in rows.
WHERE clause is used to eliminate the tuples in a relation,and HAVING clause is used to eliminate the groups in a relation.
HAVING clause is used for aggregate functions such as
MIN,MAX,COUNT,SUM .But always use GROUP BY clause before HAVING clause to minimize the error.
Both WHERE and HAVING are used to filter data.
In case of a WHERE statement, data filtering happens before you pull the data for operation.
SELECT name, age
FROM employees
WHERE age > 30;
Here the WHERE clause filters rows before the SELECT operation is performed.
SELECT department, avg(age) avg_age
FROM employees
GROUP BY department
HAVING avg_age> 35;
HAVING filters the data after the SELECT operation is performed. Here the operation of computing (aggregation) is done first and then a filter is applied to the result using a HAVING clause.
Related
SELECT name, countrycode, population, avg(population)
FROM city
WHERE population > (SELECT avg(population) FROM city);
I need to figure out what is wrong with this subquery. Personally I think it is something to do with the placement of avg(population) but I dont know exactly.
When you use an aggregation function without GROUP BY, it aggregates the entire table into a single result row. Then it makes no sense to include non-aggregated columns in the SELECT list, because it will just be one value from arbitrary rows in the table.
If you want to include the average over the entire table along with the selected rows, join with the subquery so you can refer to it in the SELECT list, instead of using the subquery in the WHERE clause.
SELECT name, countrycode, population, avg_population
FROM city
JOIN (
SELECT avg(population) AS avg_population
FROM city
) AS x ON population > avg_population
I have the following two tables:
1. Lecturers (LectID, Fname, Lname, degree).
2. Lecturers_Specialization (LectID, Expertise).
I want to find the lecturer with the most Specialization.
When I try this, it is not working:
SELECT
L.LectID,
Fname,
Lname
FROM Lecturers L,
Lecturers_Specialization S
WHERE L.LectID = S.LectID
AND COUNT(S.Expertise) >= ALL (SELECT
COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);
But when I try this, it works:
SELECT
L.LectID,
Fname,
Lname
FROM Lecturers L,
Lecturers_Specialization S
WHERE L.LectID = S.LectID
GROUP BY L.LectID,
Fname,
Lname
HAVING COUNT(S.Expertise) >= ALL (SELECT
COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);
What is the reason? Thanks.
WHERE clause introduces a condition on individual rows; HAVING clause introduces a condition on aggregations, i.e. results of selection where a single result, such as count, average, min, max, or sum, has been produced from multiple rows. Your query calls for a second kind of condition (i.e. a condition on an aggregation) hence HAVING works correctly.
As a rule of thumb, use WHERE before GROUP BY and HAVING after GROUP BY. It is a rather primitive rule, but it is useful in more than 90% of the cases.
While you're at it, you may want to re-write your query using ANSI version of the join:
SELECT L.LectID, Fname, Lname
FROM Lecturers L
JOIN Lecturers_Specialization S ON L.LectID=S.LectID
GROUP BY L.LectID, Fname, Lname
HAVING COUNT(S.Expertise)>=ALL
(SELECT COUNT(Expertise) FROM Lecturers_Specialization GROUP BY LectID)
This would eliminate WHERE that was used as a theta join condition.
First we should know the order of execution of Clauses i.e
FROM > WHERE > GROUP BY > HAVING > DISTINCT > SELECT > ORDER BY.
Since WHERE Clause gets executed before GROUP BY Clause the records cannot be filtered by applying WHERE to a GROUP BY applied records.
"HAVING is same as the WHERE clause but is applied on grouped records".
first the WHERE clause fetches the records based on the condition then the GROUP BY clause groups them accordingly and then the HAVING clause fetches the group records based on the having condition.
HAVING operates on aggregates. Since COUNT is an aggregate function, you can't use it in a WHERE clause.
Here's some reading from MSDN on aggregate functions.
WHERE clause can be used with SELECT, INSERT, and UPDATE statements, whereas HAVING can be used only with SELECT statement.
WHERE filters rows before aggregation (GROUP BY), whereas HAVING filter groups after aggregations are performed.
Aggregate function cannot be used in WHERE clause unless it is in a subquery contained in HAVING clause, whereas aggregate functions can be used in HAVING clause.
Source
Didn't see an example of both in one query. So this example might help.
/**
INTERNATIONAL_ORDERS - table of orders by company by location by day
companyId, country, city, total, date
**/
SELECT country, city, sum(total) totalCityOrders
FROM INTERNATIONAL_ORDERS with (nolock)
WHERE companyId = 884501253109
GROUP BY country, city
HAVING country = 'MX'
ORDER BY sum(total) DESC
This filters the table first by the companyId, then groups it (by country and city) and additionally filters it down to just city aggregations of Mexico. The companyId was not needed in the aggregation but we were able to use WHERE to filter out just the rows we wanted before using GROUP BY.
You can not use where clause with aggregate functions because where fetch records on the basis of condition, it goes into table record by record and then fetch record on the basis of condition we have give. So that time we can not where clause. While having clause works on the resultSet which we finally get after running a query.
Example query:
select empName, sum(Bonus)
from employees
order by empName
having sum(Bonus) > 5000;
This will store the resultSet in a temporary memory, then having clause will perform its work. So we can easily use aggregate functions here.
1.
We can use aggregate function with HAVING clause not by WHERE clause e.g. min,max,avg.
2.
WHERE clause eliminates the record tuple by tuple
HAVING clause eliminates entire group from the collection of group
Mostly HAVING is used when you have groups of data and WHERE is used when you have data in rows.
WHERE clause is used to eliminate the tuples in a relation,and HAVING clause is used to eliminate the groups in a relation.
HAVING clause is used for aggregate functions such as
MIN,MAX,COUNT,SUM .But always use GROUP BY clause before HAVING clause to minimize the error.
Both WHERE and HAVING are used to filter data.
In case of a WHERE statement, data filtering happens before you pull the data for operation.
SELECT name, age
FROM employees
WHERE age > 30;
Here the WHERE clause filters rows before the SELECT operation is performed.
SELECT department, avg(age) avg_age
FROM employees
GROUP BY department
HAVING avg_age> 35;
HAVING filters the data after the SELECT operation is performed. Here the operation of computing (aggregation) is done first and then a filter is applied to the result using a HAVING clause.
I have this query:
select count(name) as nr
from team where city='ny' and name=ANY
(select teamName from contract where playerCode=ANY
(select code from player where name='X' and surname='Y'));
I don't understand why the count() function doesn't count the duplicates even if there is no distinct clause.
These are the tables:
Player(code, name, surname)
Contract(id, playerCode, teamName, year)
Team(name, city)
With this integrity constraints:
Contract(playerCode)-->Player(code)
Contract(teamName)-->Team(name)
The query extracts the number of teams of NY city which have a contract with a player named X,Y.
Thanks.
Your query will count duplicates. My guess is that you are expecting duplicates from multiple matches in the subquery. However, the subquery is not a JOIN and so will not be duplicating results from the team table, regardless of how many matches there are in the subquery.
Each row in the team table will only have the WHERE conditions evaluated once, and will be included (once) based on whether those conditions are satisfied.
Assuming there are no NULL values in team.name, if you SELECT COUNT(name), COUNT(*) both fields should have the same value.
I have the following two tables:
1. Lecturers (LectID, Fname, Lname, degree).
2. Lecturers_Specialization (LectID, Expertise).
I want to find the lecturer with the most Specialization.
When I try this, it is not working:
SELECT
L.LectID,
Fname,
Lname
FROM Lecturers L,
Lecturers_Specialization S
WHERE L.LectID = S.LectID
AND COUNT(S.Expertise) >= ALL (SELECT
COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);
But when I try this, it works:
SELECT
L.LectID,
Fname,
Lname
FROM Lecturers L,
Lecturers_Specialization S
WHERE L.LectID = S.LectID
GROUP BY L.LectID,
Fname,
Lname
HAVING COUNT(S.Expertise) >= ALL (SELECT
COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);
What is the reason? Thanks.
WHERE clause introduces a condition on individual rows; HAVING clause introduces a condition on aggregations, i.e. results of selection where a single result, such as count, average, min, max, or sum, has been produced from multiple rows. Your query calls for a second kind of condition (i.e. a condition on an aggregation) hence HAVING works correctly.
As a rule of thumb, use WHERE before GROUP BY and HAVING after GROUP BY. It is a rather primitive rule, but it is useful in more than 90% of the cases.
While you're at it, you may want to re-write your query using ANSI version of the join:
SELECT L.LectID, Fname, Lname
FROM Lecturers L
JOIN Lecturers_Specialization S ON L.LectID=S.LectID
GROUP BY L.LectID, Fname, Lname
HAVING COUNT(S.Expertise)>=ALL
(SELECT COUNT(Expertise) FROM Lecturers_Specialization GROUP BY LectID)
This would eliminate WHERE that was used as a theta join condition.
First we should know the order of execution of Clauses i.e
FROM > WHERE > GROUP BY > HAVING > DISTINCT > SELECT > ORDER BY.
Since WHERE Clause gets executed before GROUP BY Clause the records cannot be filtered by applying WHERE to a GROUP BY applied records.
"HAVING is same as the WHERE clause but is applied on grouped records".
first the WHERE clause fetches the records based on the condition then the GROUP BY clause groups them accordingly and then the HAVING clause fetches the group records based on the having condition.
HAVING operates on aggregates. Since COUNT is an aggregate function, you can't use it in a WHERE clause.
Here's some reading from MSDN on aggregate functions.
WHERE clause can be used with SELECT, INSERT, and UPDATE statements, whereas HAVING can be used only with SELECT statement.
WHERE filters rows before aggregation (GROUP BY), whereas HAVING filter groups after aggregations are performed.
Aggregate function cannot be used in WHERE clause unless it is in a subquery contained in HAVING clause, whereas aggregate functions can be used in HAVING clause.
Source
Didn't see an example of both in one query. So this example might help.
/**
INTERNATIONAL_ORDERS - table of orders by company by location by day
companyId, country, city, total, date
**/
SELECT country, city, sum(total) totalCityOrders
FROM INTERNATIONAL_ORDERS with (nolock)
WHERE companyId = 884501253109
GROUP BY country, city
HAVING country = 'MX'
ORDER BY sum(total) DESC
This filters the table first by the companyId, then groups it (by country and city) and additionally filters it down to just city aggregations of Mexico. The companyId was not needed in the aggregation but we were able to use WHERE to filter out just the rows we wanted before using GROUP BY.
You can not use where clause with aggregate functions because where fetch records on the basis of condition, it goes into table record by record and then fetch record on the basis of condition we have give. So that time we can not where clause. While having clause works on the resultSet which we finally get after running a query.
Example query:
select empName, sum(Bonus)
from employees
order by empName
having sum(Bonus) > 5000;
This will store the resultSet in a temporary memory, then having clause will perform its work. So we can easily use aggregate functions here.
1.
We can use aggregate function with HAVING clause not by WHERE clause e.g. min,max,avg.
2.
WHERE clause eliminates the record tuple by tuple
HAVING clause eliminates entire group from the collection of group
Mostly HAVING is used when you have groups of data and WHERE is used when you have data in rows.
WHERE clause is used to eliminate the tuples in a relation,and HAVING clause is used to eliminate the groups in a relation.
HAVING clause is used for aggregate functions such as
MIN,MAX,COUNT,SUM .But always use GROUP BY clause before HAVING clause to minimize the error.
Both WHERE and HAVING are used to filter data.
In case of a WHERE statement, data filtering happens before you pull the data for operation.
SELECT name, age
FROM employees
WHERE age > 30;
Here the WHERE clause filters rows before the SELECT operation is performed.
SELECT department, avg(age) avg_age
FROM employees
GROUP BY department
HAVING avg_age> 35;
HAVING filters the data after the SELECT operation is performed. Here the operation of computing (aggregation) is done first and then a filter is applied to the result using a HAVING clause.
Why can't I write queries like
select *
from STAFF
having Salary > avg(salary);
in SQL?
PS: to find staff members with a salary = avg salary, I need to write
select *
from STAFF
having Salary > (select avg(Salary) from STAFF);
Is there any other way to do this?
An aggregate may not appear in the WHERE clause unless it is in a subquery contained in a HAVING clause or a select list, and the column being aggregated is an outer reference.
The functions SUM(),AVG(),MIN(),MAX(),COUNT(),etc are called aggregate functions. Read more here.
Example using WHERE clause :
select *
from staff
where salary > (select avg(salary) from staff)
See example in SQL Fiddle.
Example using HAVING clause :
select deptid,COUNT(*) as TotalCount
from staff
group by deptid
having count(*) >= 2
See example in SQL Fiddle.
Where can we use having clause:
Having clause specifies a search condition for a group or an aggregate. HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause.
Read more here.