I have two tables: 1) Places 2) Reviews
Table examples are below:
PLACES
ID | NAME
============
1 | Joe
2 | Cat
3 | Dog
REVIEWS
PLACE_ID | REVIEW_ID| REVIEW_CONTENT
====================================
1 | 1000 | "it's good"
1 | 1001 | "aweful place"
3 | 1002 | "good place"
PLACE_ID is my foreign key and I want to count number of review contents per each ID in PLACES table.
As you can see,
there are 2 review contents in REVIEWS table for place id 1 ("Joe")
there are 0 review contents in REVIEWS table for place id 2 ("Cat")
there are 1 review contents in REVIEWS table for place id 3 ("Dog")
The result should look like
RESULT
PLACE_ID | NAME | COUNT
=======================
1 | Joe | 2
2 | Cat | 0
3 | Dog | 1
Can someone please help how to count number of rows (e.g number of review contents) that has same foreign key (e.g. PLACE_ID), given two tables?
This is basic SQL. Please do some reading on simple aggregations.
SELECT P.ID as PLACE_ID,
P.NAME as NAME,
COUNT(R.ID) as COUNT
FROM PLACES P
LEFT JOIN REVIEWS R
ON P.ID = R.PLACE_ID
You can try the below - using left join and aggregation
SELECT p.id, p.name,count(r.id) as cnt
from place p left join reviews ON p.id = r.place_id
group by p.id, p.name
Simple Join both the tables and perform a aggregation to count the number of reviews for each ID available in Place table. You can find the code below.
Select A.PLACE_ID,
A.Name,
count(REVIEW_ID) COUNT
From Places A
Left Join Reviews B
on A.ID = B.PLACE_ID
group by A.PLACE_ID,
A.Name
Related
I'm trying to find the query in order to match similar customers.
To simplify the situation consider this scenario:
I have a table which contains a customer name and product purchased.
customer name can have multiple purchases of same and different products.
So firstly I can take distinct customer name and product name, so I see all customers and all products they purchased at least once.
Now I want a query to show me a sort of matching customers, according to the product they both purchased, so I want to count the similar products they purchased.
So I want to see for each pair of customers (pairing all the table) the amount of similar product they purchased.
Lets say the raw data is:
CustomerName | ProductName
A | 1
A | 2
A | 1
A | 3
B | 1
B | 2
B | 4
C | 2
Then I want to see the result of:
CustomerName1 | CustomerName2 | CountSimilarity
A | B | 2
A | C | 1
B | C | 1
And so on for all pairs of customers that have at least 1 similar product purchasing
Any suggestions how to approach this query?
The environment is SQL Server.
Thanks
Here is a self join approach:
SELECT t1.CustomerName, t2.CustomerName, COUNT(*) AS CountSimilarity
FROM yourTable t1
INNER JOIN yourTable t2
ON t1.ProductName = t2.ProductName
WHERE
t1.CustomerName < t2.CustomerName
GROUP BY
t1.CustomerName, t2.CustomerName;
Two records are joined together above if their products match. Note that the inequality in the WHERE clause ensures that customer pairs do not appear in duplicate.
I Have a query that finds a table, here's an example one.
Name |Age |Hair |Happy | Sad |
Jon | 15 | Black |NULL | NULL|
Kyle | 18 |Blonde |YES |NULL |
Brad | 17 | Blue |NULL |YES |
Name and age come from one table in a database, hair color comes from a second which is joined, and happy and sad come from a third table.My goal would be to make the first line of the chart like this:
Name |Age |Hair |Happy |Sad |
Jon | 15 |Black |Yes |Yes |
Basically I want to get rid of the rows under the first and get the non NULL data joined to the right. The problem is that there is no column where the Yes values are on the Jon row, so I have no idea how to get them there. Any suggestions?
PS. With the data I am using I can't just put a 'YES' in the 'Jon' row and call it a day, I would need to find the specific value from the lower rows and somehow get that value in the boxes that are NULL.
Do you just want COALESCE()?
COALESCE(Happy, 'Yes') as happy
COALESCE() replaces a NULL value with another value.
If you want to join on a NULL value work with nested selects. The inner select gets an Id for NULLs, the outer select joins
select COALESCE(x.Happy, yn_table.description) as happy, ...
from
(select
t1.Happy,
CASE WHEN t1.Happy is null THEN 1 END as happy_id
from t1 ...) x
left join yn_table
on x.xhappy_id = yn_table.id
If you apply an ORDER BY to the query, you can then select the first row relative to this order with WHERE rownum = 1. If you don't apply an ORDER BY, then the order is random.
After reading your new comment...
the sense is that in my real data the yes under the other names will be a number of a piece of equipment. I want the numbers of the equipment in one row instead of having like 8 rows with only 4 ' yes' values and the rest null.
... I come to the conclusion that this a XY problem.
You are asking about a detail you think will solve your problem, instead of explaining the problem and asking how to solve it.
If you want to store several pieces of equipment per person, you need three tables.
You need a Person table, an Article table and a junction table relating articles to persons to equip them. Let's call this table Equipment.
Person
------
PersonId (Primary Key)
Name
optional attributes like age, hair color
Article
-------
ArticleId (Primary Key)
Description
optional attributes like weight, color etc.
Equipment
---------
PersonId (Primary Key, Foreign Key to table Person)
ArticleId (Primary Key, Foreign Key to table Article)
Quantity (optional, if each person can have only one of each article, we don't need this)
Let's say we have
Person: PersonId | Name
1 | Jon
2 | Kyle
3 | Brad
Article: ArticleId | Description
1 | Hat
2 | Bottle
3 | Bag
4 | Camera
5 | Shoes
Equipment: PersonId | ArticleId | Quantity
1 | 1 | 1
1 | 4 | 1
1 | 5 | 1
2 | 3 | 2
2 | 4 | 1
Now Jon has a hat, a camera and shoes. Kyle has 2 bags and one camera. Brad has nothing.
You can query the persons and their equipment like this
SELECT
p.PersonId, p.Name, a.ArticleId, a.Description AS Equipment, e.Quantity
FROM
Person p
LEFT JOIN Equipment e
ON p.PersonId = e.PersonId
LEFT JOIN Article a
ON e.ArticleId = a.ArticleId
ORDER BY p.Name, a.Description
The result will be
PersonId | Name | ArticleId | Equipment | Quantity
---------+------+-----------+-----------+---------
3 | Brad | NULL | NULL | NULL
1 | Jon | 4 | Camera | 1
1 | Jon | 1 | Hat | 1
1 | Jon | 5 | Shoes | 1
2 | Kyle | 3 | Bag | 2
2 | Kyle | 4 | Camera | 1
See example: http://sqlfiddle.com/#!4/7e05d/2/0
Since you tagged the question with the oracle tag, you could just use NVL(), which allows you to specify a value that would replace a NULL value in the column you select from.
Assuming that you want the 1st row because it contains the smallest age:
- wrap your query inside a CTE
- in another CTE get the 1st row of the query
- in another CTE get the max values of Happy and Sad of your query (for your sample data they both are 'YES')
- cross join the last 2 CTEs.
with
cte as (
<your query here>
),
firstrow as (
select name, age, hair from cte
order by age
fetch first row only
),
maxs as (
select max(happy) happy, max(sad) sad
from cte
)
select f.*, m.*
from firstrow f cross join maxs m
You can try this:
SELECT A.Name,
A.Age,
B.Hair,
C.Happy,
C.Sad
FROM A
INNER JOIN B
ON A.Name = B.Name
INNER JOIN C
ON A.Name = B.Name
(Assuming that Name is the key columns in the 3 tables)
Let's say I have the following tables:
+-------------------------------------------+
| t_classroom |
+-------------------------------------------+
| PK | id |
| | admin_user_id |
| | name |
| | students |
+-------------------------------------------+
+-------------------------------------------+
| t_shared |
+-------------------------------------------+
| | admin_user_id |
| | classroom_id |
| | expiry |
+-------------------------------------------+
I want to write a query that will pull all classrooms that an admin_user_id has access to. In essence, I want a union of classroom rows when I search by admin_user_id in the t_classroom table as well as classroom rows when I search by admin_user_id in the t_shared table. I made the following attempt:
SELECT
id,
admin_user_id,
name,
students
FROM
t_classroom
WHERE
admin_user_id = 1
UNION ALL
SELECT
c.id,
c.admin_user_id,
c.name,
students
FROM
t_classroom c
INNER JOIN t_shared s
ON c.id = s.classroom_id
WHERE
admin_user_id = 1
Does the above look correct? Is there anything more efficient/cleaner?
Depending on how much data you have you could probably get away with just using an IN clause to look at the other table.
SELECT
c.id,
c.admin_user_id,
c.name,
c.students
FROM
t_classroom c
WHERE
c.admin_user_id = 1
OR c.id IN ( select s.classroom_id from t_shared s where s.admin_user_id = 1 )
Your union wont work because you're left-joining to the t_shared table and checking only the classroom admin user.
If you join the shared room you would also end up with duplicates and would need to distinct the result too.
Edit:
Because of the large number of rows it might be better to use an exists check on the 2nd table.
SELECT
c.id,
c.admin_user_id,
c.name,
c.students
FROM
t_classroom c
WHERE
c.admin_user_id = 1
OR EXISTS ( select 1 from t_shared s where s.classroom_id = c.id AND s.admin_user_id = 1 )
Your solution is fundamentally fine, the only two problems I can detect when eyeballing your query are:
You need to write s.admin_user_id instead of admin_user_id in the last line to avoid an error message, because there is a column of that name in both tables. Best practice is to always qualify column names with the table names.
You might want to use UNION instead of UNION ALL if you want to avoid a duplicate result row in the case that both tables have admin_user_id = 1 for the same classroom.
I am getting a bit confused with this basic SQL question, I was hoping you could give me a hint about what am I doing wrong:
I have a main table with different players:
id | name
----+-----
1 | John
2 | Paul
3 | Robert
And another table that stores their matches
date | player_home | player_away
----------+---------------+-------------
2012-03-21| 1 | 2
2012-04-10| 2 | 3
I am trying to build a query that outputs the names of both players for a given date, but the name of the same player is being repeated in both fields:
This retrieves nothing:
SELECT date, player_home, name, player_away, name
FROM games, players
WHERE date = (DATE '2012-03-21')
AND games.player_home = players.id
AND games.player_away = players.id;
And if I remove the second AND, it retrieves the same name (John) for both players:
date | player_home | name | player_away | name
----------+---------------+---------+----------------+-------
2012-03-21| 1 | John | 2 | John
When the name in the second "name" field should be "Paul".
What is the right way to build this query?
Thanks in advance
You need to join the players table twice and use different alias names for the tables
SELECT g.date, p1.name as home_name, p2.name as away_name
FROM games g
JOIN players p1 ON g.player_home = p1.id
JOIN players p2 ON g.player_away = p2.id
WHERE g.date = (DATE '2012-03-21')
SELECT date, player_home, players_home.name, player_away, player_away.name
FROM games
left outer join players players_home on player_home=players.id
left outer join players players_away on player_away=players.id
WHERE date = (DATE '2012-03-21') ;
I have 2 tables
Bid_customer
|bidkey | customerkey
| 1 | 1
| 1 | 2
| 1 | 3
customer_groups
| groupkey | customerkey
| 1 | 1
| 1 | 2
| 1 | 3
What I'm trying to get is a result that will look like
| bidkey | groupkey
| 1 | 1
I've tried a cursor and joins but just don't seem to be able to get what i need any ideas or suggestions
EDIT: customers can belong to more that one group also
I am not sure who meaningful your sample data is. However following is a simple example.
Query:
select distinct b.bidkey, g.gkey
from bidcus b
inner join cusgroup g
on
b.cuskey = g.cuskey
and g.gkey = 10;
Results:
BIDKEY GKEY
1 10
Reference: SQLFIDDLE
In order to have a working Many-to-Many relationship in a database you need to have an intermediary table that defines the relationship so you do not get duplicates or mismatched values.
This select statement will join all bids with all groups because the customer matches.
Select bidkey, groupkey
From customer_groups
Inner Join bid_customer
Where customer_groups.customerkey = Bid_customer.customerkey
Hers is a sample Many to Many Relationship:
For your question:
You will need another table that joins the data. For example, GroupBids
customer_groups and bid_customer would have a one-to-many relationship with GroupBids
You would then do the following select to get your data.
Select bidkey, groupkey
From bid_customer
inner join GroupBids
ON bid_customer.primarykey = GroupBids.idBidKey
inner join customer_groups
ON customer_groups.primarykey = GroupBids.idCustomerGroupkey
This would make sure only related groups and bids are returned