I'm writing some SQL code based on my tables but don't want to miss any edge cases. I'm wondering how do you check if there's a one to many relationship between two tables in SSMS
SELECT *
FROM Houses
JOIN Addresses
on Houses.Id = Addresses.HouseId
Unfortunately the data when queried doesn't give me any insight.
What I tried to do:
Checked table dependencies but that didn't give me any insight. It shows addresses are dependencies but no relationship details.
May I ask, is it possible to determine if one to one via SSMS?
You can run a query to determine how many values each house in the two tables. Well, in this case, we'll assume it is the primary key of houses and the values of HouseId are valid ids:
SELECT num_addresses, COUNT(*)
FROM (SELECT h.Id, COUNT(a.HouseId) as num_addresses
FROM Houses h LEFT JOIN
Addresses a
ON h.Id = a.HouseId
GROUP BY h.Id
) ha
GROUP BY num_addresses;
Then interpret the results:
If the only row returned has num_addresses of 1, then you have a 1-1 relationship.
If two rows are returns with values of 0 and 1, then you have a 1/0-1 relationship.
If multiple rows are returned and the minimum is 1 then you have a 1-n.
If multiple rows are returned and the minimum value is 0, then you have a 0-n.
You could extend this for more general relationships, but this answers the question you asked here.
The relationship between tables is shown by things called 'DATABASE - DIAGRAM'
Here is document about how to create database - diagram in SSMS.
How to create database diagram SSMS
Gordon Linoff response is perfect for the helicopter view -
To get all records side by side simply select all columns from both tables side by side with the left join
SELECT Houses.*, Address.*
FROM Houses
left JOIN Addresses on Addresses.HouseId = Houses.Id
union all
SELECT Houses.*, Address.*
FROM Addresses
left JOIN Houses on Houses.Id = Addresses.HouseId
Microsoft have implemented a reduced code method to do the same but I have not explored it so could not comment
https://learn.microsoft.com/en-us/sql/odbc/reference/develop-app/outer-joins?view=sql-server-ver15
What will be apparent is you will see Houses.ID and Addresses.HouseID side by side - An empty value in either means that the relevant table does not have a record for the relevant HouseID or ID
You can then cherry pick those edge case records as you see fit
Related
I need to brind a lot of columns from several tables using LEFT JOIN. My starting point is the orders table and I bring the vendor name from the "Address_table". Then I add another table with order details and then the shipping information of each order detail.
My problem is that I need to bring a different record from "Address_table" to refer onether id's detailed in shipment table as of "origin_id" and "destination_id".
In other words, "address_id", "origin_id" and "destination_id" are all records from "Address_table". I brought the first one related to the vendor, how can I retrieve the other two?
Example
Thanks in advance
Your question is not exactly clear in terms of the tables and their relationships. It is, however, clear what the problem is. You need to join against the same table twice using different columns.
In order to do that you need to use table aliases. For example, you can do:
select *
from shipment s
left join address_table a on a.address_id = s.origin_id
left join address_table b on b.address_id = s.destination_id
In this example the table address_table is joined twice against the table shipment; the first time we use a as an alias, the second time b. This way you can differentiate how to pick the right columns and make the joins work as you need them to.
I have two tables: violator_state and violator_tags
violator_state:
m_state_id
is_violating
m_translatedid
m_tag
m_violator_tag
This table holds the "tags" which has an unchanging row count of 10 in this case. The purpose is to list out each tag present, connect the full tag address (m_violator_tag) with its shorthand name (m_tag) and state whether it is in "violation". I need to use this table as reference because of the link between m_violator_tag and m_tag.
violator_tags
m_violator_id
m_eval_time_from
m_eval_time_to
m_tag
m_tag_peers
m_tag_position
This table is constantly having new rows added to it holding the information of what tags are in violation with a specific tag. So it would show T6 in violation with T1,T2,T9 ect.
I am looking to create a query which joins the two tables to show only the most recently updated (largest m_eval_time_from) for each tag.
I am using the following query to join the two tables but I expect m_translatedid and m_tag to match but they do not. Unsure why.
SELECT violator_state.m_violator_tag, violator_state.is_violating, violator_state.m_translatedid, violator_tags.m_tag, violator_tags.m_eval_time_to, violator_tags.m_tag_peers,
violator_tags.m_tag_position, violator_tags.m_eval_time_from
FROM violator_tags CROSS JOIN
violator_state
Violation_state table
violation_tags table
results of my (incorrect) query
Any suggestions on what I should try?
Your CROSS JOIN will give you a cartesian product where EVERY row in the first table is paired with ALL the rows in the second table e.g. if you have 10 rows in each, you will get 10 x 10 = 100 rows in the result! I believe you need to join the tables on the m_tag column and select the violator_tags row with the latest date. The query below should do this for you (though you haven't provided your question in a manner that makes it easy for me to double-check my code - see the link provided by a_horse_with_no_name for more on this or use a website like db-fiddle to set up your example).
SELECT vs.m_violator_tag,
vs.is_violating,
vs.m_translatedid,
vt.m_tag,
vt.m_eval_time_to,
vt.m_tag_peers,
vt.m_tag_position,
vt.m_eval_time_from
FROM violator_tags vt
JOIN violator_state vs
ON vt.m_tag = vs.m_tag
AND vt.m_eval_time_from = (SELECT MAX(vt.m_eval_time_from)
FROM violator_tags
WHERE m_tag = vt.m_tag)
http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html
and
https://web.archive.org/web/20120621231245/http://www.khankennels.com/blog/index.php/archives/2007/04/20/getting-joins/
have been very helpful in learning the basics of joins using Venn diagrams. But I am wondering how you would apply that same thinking to a query that has more than one join.
Let's say I have 3 tables:
Employees
EmployeeID
FullName
EmployeeTypeID
EmployeeTypes (full time, part time, etc.)
EmployeeTypeID
TypeName
InsuranceRecords
InsuranceRecordID
EmployeeID
HealthInsuranceNumber
Now, I want my final result set to include data from all three tables, in this format:
EmployeeID | FullName | TypeName | HealthInsuranceNumber
Using what I learned from those two sites, I can use the following joins to get all employees, regardless of whether or not their insurance information exists or not:
SELECT
Employees.EmployeeID, FullName, TypeName, HealthInsuranceNumber
FROM Employees
INNER JOIN EmployeeTypes ON Employees.EmployeeTypeID = EmployeeTypes.EmployeeTypeID
LEFT OUTER JOIN InsuranceRecords ON Employees.EmployeeID = InsuranceRecords.EmployeeID
My question is, using the same kind of Venn diagram pattern, how would the above query be represented visually? Is this picture accurate?
I think it is not quite possible to map your example onto these types of diagrams for the following reason:
The diagrams here are diagrams used to describe intersections and unions in set theory. For the purpose of having an overlap as depicted in the diagrams, all three diagrams need to contain elements of the same type which by definition is not possible if we are dealing with three different tables where each contains a different type of (row-)object.
If all three tables would be joined on the same key then you could identify the values of this key as the elements the sets contain but since this is not the case in your example these kind of pictures are not applicable.
If we do assume that in your example both joins use the same key, then only the green area would be the correct result since the first join restricts you to the intersection of Employees and Employee types and the second join restricts you the all of Employees and since both join conditions must be true you would get the intersection of both of the aforementioned sections which is the green area.
Hope this helps.
That is not an accurate set diagram (either Venn or Euler). There are no entities which are members of both Employees and Employee Types. Even if your table schema represented some kind of table-inheritance, all the entities would still be in a base table.
Jeff's example on the Coding Horror blog only works with like entities i.e. two tables containing the same entities - technically a violation of normalization - or a self-join.
Venn diagrams can accurately depict scenarios like:
-- The intersection lens
SELECT *
FROM table
WHERE condition1
AND condition2
-- One of the lunes
SELECT *
FROM table
WHERE condition1
AND NOT condition2
-- The union
SELECT *
FROM table
WHERE condition1
OR condition2
I have 2 tables with a many-to-any relationship. For the example we will call the tables "Guys" and Girls" There is a junction table that contains the related primary keys...who has dated who.
If I want to find all the girls that Guy 1 has dated, I do a select on the junction table selecting all girls with guys.ID. This give me a RecordSet. Now to find the names of the girls, I need to select from the girls table a row using the key from each RecordSet row.
Isn't there an easier way? Since I've defined the relationships in Access I would think that there must be a way to build a single query. How do I do that?
SELECT girls.name
FROM (guys
INNER JOIN junct ON guys.guyID = junct.guyID)
INNER JOIN girls ON junct.girlID = girls.girlID
WHERE guys.guyID = [whatever id you're looking for]
It seems to me that there are two scenarios in which to use JOINs:
When data would otherwise be duplicated
When data from one query would otherwise be used in another query
Are these scenarios right? Are there any other scenarios in which to use JOIN?
EDIT: I think I've miscommunicated. I understand how a JOIN works, what I'm not so sure about is when to use one.
JOINS are used to JOIN tables together with related information.
Tipical situations are where you have lets say
A user table where the user has specific security settings. The join would be used such that you can determine which settings the user has.
Users
-UserID
-UserName
UserSecurityRoles
-UserID
-SecurityRoleID
SecurityRoles
-SecurityRoleID
-SecurityRole
SELECT *
FROM Users u INNER JOIN
UserSecurityRoles usr ON u.UserID = usr.UserID INNER JOIN
SecurityRoles sr ON usr.SecurityRoleID = sr.SecurityRoleID
WHERE sr.SecurityRole = 'Admin'
LEFT joins will be used in the cases where you wish to retrieve all the data from the table in the left hand side, and only data from the right that match.
JOINS are used when you start normalizing your table structure. You can crete a table with 100s on columns, where a lot of the data could possibly be NULL, or you can normalize the tables, such that you avoid having too many columns with null values, where you group the appropriate data into table structures.
The answer to this Question has a VERY good link that has graphical display of using JOINs
JOINS are used to return data that is related in a relational database. Data can be related in 3 ways
One to Many relationship (A Person can have many Transactions)
Many to Many relationship (A Doctor can have many Patients, but a Patient can have more than one Doctor)
One to One relationship (One Person can have exactly one Passport number)
JOINS come in various flavours:
AN INNER JOIN will return data from both tables where the keys in each table match
A LEFT JOIN or RIGHT JOIN will return all the rows from one table and matching data from the other table
A CROSS JOIN will return the product of each table
You use joins when you need information from more than one table :)