I'm taking a database course this semester, and we're learning SQL. I understand most simple queries, but I'm having some difficulty using the count aggregate function.
I'm supposed to relate an advertisement number to a property number to a branch number so that I can tally up the amount of advertisements by branch number and compute their cost. I set up what I think are two appropriate new views, but I'm clueless as to what to write for the select statement. Am I approaching this the correct way? I have a feeling I'm over complicating this bigtime...
with ad_prop(ad_no, property_no, overseen_by) as
(select a.ad_no, a.property_no, p.overseen_by
from advertisement as a, property as p
where a.property_no = p.property_no)
with prop_branch(property_no, overseen_by, allocated_to) as
(select p.property_no, p.overseen_by, s.allocated_to
from property as p, staff as s
where p.overseen_by = s.staff_no)
select distinct pb.allocated_to as branch_no, count( ??? ) * 100 as ad_cost
from prop_branch as pb, ad_prop as ap
where ap.property_no = pb.property_no
group by branch_no;
Any insight would be greatly appreciated!
You could simplify it like this:
advertisement
- ad_no
- property_no
property
- property_no
- overseen_by
staff
- staff_no
- allocated_to
SELECT s.allocated_to AS branch, COUNT(*) as num_ads, COUNT(*)*100 as ad_cost
FROM advertisement AS a
INNER JOIN property AS p ON a.property_no = p.property_no
INNER JOIN staff AS s ON p.overseen_by = s.staff_no
GROUP BY s.allocated_to;
Update: changed above to match your schema needs
You can condense your WITH clauses into a single statement. Then, the piece I think you are missing is that columns referenced in the column definition have to be aggregated if they aren't included in the GROUP BY clause. So you GROUP BY your distinct column then apply your aggregation and math in your column definitions.
SELECT
s.allocated_to AS branch_no
,COUNT(a.ad_no) AS ad_count
,(ad_count * 100) AS ad_cost
...
GROUP BY s.allocated_to
i can tell you that you are making it way too complicated. It should be a select statement with a couple of joins. You should re-read the chapter on joins or take a look at the following link
http://www.sql-tutorial.net/SQL-JOIN.asp
A join allows you to "combine" the data from two tables based on a common key between the two tables (you can chain more tables together with more joins). Once you have this "joined" table, you can pretend that it is really one table (aliases are used to indicate where that column came from). You understand how aggregates work on a single table right?
I'd prefer not to give you the answer so that you can actually learn :)
Related
I have a distinct list of part numbers from one table. It is basically a table that contains a record of all the company's part numbers. I want to add columns that will pull data from different tables but only pertaining to the part number on that row of the distinct part list.
For example: if I have part A, B, C from the unique part list I want to add columns for Purchase quantity, repair quantity, loan quantity, etc... from three totally unique tables.
So it's almost like I need 3 subqueries that will sum of that data from the different tables for each part.
Can anybody steer me in the direction of how to do this? Please and thank you so much!
One method is correlated subqueries. Something like this:
select p.*,
(select count(*)
from purchases pu
where pu.part_id = p.part_id
) as num_purchases,
(select count(*)
from repairs r
where r.part_id = p.part_id
) as num_repairs,
(select count(*)
from loans l
where l.part_id = p.part_id
) as num_loans
from parts p;
Another option is joins with aggregation before the join. Or lateral joins (which are quite similar to correlated subqueries).
I am trying to count all the values in one column and then multiply this number by a value in a different table. So far I have:
SELECT CLUB_FEE * COUNT(MEMBER_ID) AS VALUE
FROM CLUB, SUBSCRIPTION
WHERE CLUB_ID = 'CLUB1';
This is not working however, can anyone please help?
I also need help doing this for multiple clubs. Is it possible to do it all in one statement for all clubs and then get the average?
Presumably, you intend something like this:
SELECT MAX(c.CLUB_FEE) * COUNT(MEMBER_ID) AS VALUE
FROM CLUB c JOIN
SUBSCRIPTION s
ON c.CLUB_ID = s.CLUB_ID
WHERE c.CLUB_ID = 'CLUB1';
You can also write this as:
SELECT SUM(c.CLUB_FEE) AS VALUE
FROM CLUB c JOIN
SUBSCRIPTION s
ON c.CLUB_ID = s.CLUB_ID
WHERE c.CLUB_ID = 'CLUB1';
I thought the first version would be clearer, because the OP specifies COUNT() in the question.
If you want it for all clubs that have subscribers:
SELECT SUM(c.CLUB_FEE) AS VALUE
FROM CLUB c JOIN
SUBSCRIPTION s
ON c.CLUB_ID = s.CLUB_ID
GROUP BY c.CLUB_ID;
From inspecting the explain plans, it seems the following version may be a bit more efficient (since it avoids a join and uses only one aggregation). If you need this for ALL clubs at the same time, then probably all solutions will have the same "optimizer cost" (they will all do a join at some point).
select club_fee * (select count(member_id) from subscription where club_id = 'CLUB1')
from club
where club_id = 'CLUB1'
So now the only aggregate function is pushed into a subquery and the rest does not need either a join or another aggregate function.
Of course, this only matters if performance is important; it may very well not be.
I'm pretty sure this works universally across various SQL implementations. Suppose I have many-to-many relationship between 2 tables:
Customer: id, name
has many:
Order: id, description, total_price
and this relationship is in a junction table:
Customer_Order: order_date, customer_id, order_id
Now I want to write SQL query to join all of these together, mentioning the customer's name, the order's description and total price and the order date:
SELECT name, description, total_price FROM Customer
JOIN Customer_Order ON Customer_Order.customer_id = Customer.id
JOIN Order = Order.id = Customer_Order.order_id
This is all well and good. This query will also work if we change the order so it's FROM Customer_Order JOIN Customer or put the Order table first. Why is this the case? Somewhere I've read that JOIN works like an arithmetic operator (+, * etc.) taking 2 operands and you can chain operator together so you can have: 2+3+5, for example. Following this logic, first we have to calculate 2+3 and then take that result and add 5 to it. Is it the same with JOINs?
Is it that behind the hood, the first JOIN must first be completed in order for the second JOIN to take place? So basically, the first JOIN will create a table out of the 2 operands left and right of it. Then, the second JOIN will take that resulting table as its left operand and perform the usual joining. Basically, I want to understand how multiple JOINs work behind the hood.
In many ways I think ORMs are the bane of modern programming. Unleashing a barrage of underprepared coders. Oh well diatribe out of the way, You're asking a question about set theory. THere are potentially other options that center on relational algebra but SQL is fundamentally set theory based. here are a couple of links to get you started
Using set theory to understand SQL
A visual explanation of SQL
Say you have these tables:
PHARMACY(**___id_pharmacy___**, name, addr, tel)
PHARMACIST(**___Insurance_number___**, name, surname, qualification, **id_pharmacy**)
SELLS(**___id_pharmacy___**, **___name___**, price)
DRUG(**___Name___**, chem_formula, **id_druggistshop**)
DRUGGISTSHOP(**___id_druggistshop___**, name, address)
I think this will be more specific.
So, I'm trying to construct an SQL statement, in which I will fetch the data from id_pharmacy and name FROM PHARMACY, the insurance_number, name, and surname columns from PHARMACIST, for all the pharmacies that sell the drug called Kronol.
And that's basically it. I know I'm missing the relationships in the code I wrote previously.
Note: Those column names which have underscores left and right to them are underlined(Primary keys).
The query you've written won't work in any DBMS that I know of.
You'll most likely want to use some combination of JOINs.
Since the exact schema isn't provided, consider this pseudo code, but hopefully it will get you on the right track.
SELECT PH.Ph_Number, PH.Name, PHCL.Ins_Number, PHCL.Name, PHCL.Surname
FROM PH
INNER JOIN PHCL ON PHCL.PH_Number = PH.Ph_Number
INNER JOIN MLIST ON MLIST.PH_Number = PH.PH_Number
WHERE MLIST.Name = "Andy"
I've obviously assumed some relationships between tables that may or may not exist, but hopefully this will be pretty close. The UNION operator won't work because you're selecting different columns and a different number of columns from the various tables. This is the wrong approach all together for what you're trying to do. It's also worth mentioning that a LEFT JOIN may or may not be a better option for you, depending on the exact requirements you're trying to meet.
Ok, try this query:
SELECT A.id_pharmacy, A.name AS PharmacyName, B.Insurance_number,
B.name AS PharmacistName, B.surname AS PharmacistSurname
FROM PHARMACY A
LEFT JOIN PHARMACIST B
ON A.id_pharmacy = B.id_pharmacy
WHERE A.id_pharmacy IN (SELECT id_pharmacy FROM SELLS WHERE name = 'Kronol')
Of all the thousands of queries I've written, I can probably count on one hand the number of times I've used a non-equijoin. e.g.:
SELECT * FROM tbl1 INNER JOIN tbl2 ON tbl1.date > tbl2.date
And most of those instances were probably better solved using another method. Are there any good/clever real-world uses for non-equijoins that you've come across?
Bitmasks come to mind. In one of my jobs, we had permissions for a particular user or group on an "object" (usually corresponding to a form or class in the code) stored in the database. Rather than including a row or column for each particular permission (read, write, read others, write others, etc.), we would typically assign a bit value to each one. From there, we could then join using bitwise operators to get objects with a particular permission.
How about for checking for overlaps?
select ...
from employee_assignments ea1
, employee_assignments ea2
where ea1.emp_id = ea2.emp_id
and ea1.end_date >= ea2.start_date
and ea1.start_date <= ea1.start_date
Whole-day inetervals in date_time fields:
date_time_field >= begin_date and date_time_field < end_date_plus_1
Just found another interesting use of an unequal join on the MCTS 70-433 (SQL Server 2008 Database Development) Training Kit book. Verbatim below.
By combining derived tables with unequal joins, you can calculate a variety of cumulative aggregates. The following query returns a running aggregate of orders for each salesperson (my note - with reference to the ubiquitous AdventureWorks sample db):
select
SH3.SalesPersonID,
SH3.OrderDate,
SH3.DailyTotal,
SUM(SH4.DailyTotal) RunningTotal
from
(select SH1.SalesPersonID, SH1.OrderDate, SUM(SH1.TotalDue) DailyTotal
from Sales.SalesOrderHeader SH1
where SH1.SalesPersonID IS NOT NULL
group by SH1.SalesPersonID, SH1.OrderDate) SH3
join
(select SH1.SalesPersonID, SH1.OrderDate, SUM(SH1.TotalDue) DailyTotal
from Sales.SalesOrderHeader SH1
where SH1.SalesPersonID IS NOT NULL
group by SH1.SalesPersonID, SH1.OrderDate) SH4
on SH3.SalesPersonID = SH4.SalesPersonID AND SH3.OrderDate >= SH4.OrderDate
group by SH3.SalesPersonID, SH3.OrderDate, SH3.DailyTotal
order by SH3.SalesPersonID, SH3.OrderDate
The derived tables are used to combine all orders for salespeople who have more than one order on a single day. The join on SalesPersonID ensures that you are accumulating rows for only a single salesperson. The unequal join allows the aggregate to consider only the rows for a salesperson where the order date is earlier than the order date currently being considered within the result set.
In this particular example, the unequal join is creating a "sliding window" kind of sum on the daily total column in SH4.
Dublicates;
SELECT
*
FROM
table a, (
SELECT
id,
min(rowid)
FROM
table
GROUP BY
id
) b
WHERE
a.id = b.id
and a.rowid > b.rowid;
If you wanted to get all of the products to offer to a customer and don't want to offer them products that they already have:
SELECT
C.customer_id,
P.product_id
FROM
Customers C
INNER JOIN Products P ON
P.product_id NOT IN
(
SELECT
O.product_id
FROM
Orders O
WHERE
O.customer_id = C.customer_id
)
Most often though, when I use a non-equijoin it's because I'm doing some kind of manual fix to data. For example, the business tells me that a person in a user table should be given all access roles that they don't already have, etc.
If you want to do a dirty join of two not really related tables, you can join with a <>.
For example, you could have a Product table and a Customer table. Hypothetically, if you want to show a list of every product with every customer, you could do somthing like this:
SELECT *
FROM Product p
JOIN Customer c on p.SKU <> c.SSN
It can be useful. Be careful, though, because it can create ginormous result sets.