I have two tables that I am joining, the EventRequest table and the Customer table. The Customer.CustNo is a primary key and the EventRequest.CustNo is a foreign key:
SELECT EventNo, DateHeld, Customer.CustNo, CustName
FROM EventRequest
INNER JOIN Customer ON EventRequest.CustNo = Customer.CustNo
My question is, is there any difference between using Customer.CustNo instead of EventRequst.CustNo in the SELECT statement when doing an INNER JOIN? Is one preferred over the other and why?
There is no difference, but I prefer use the field of the main table.
Or the field from the table that you are retrieving more fields.
It does not matter which table's column you reference to make an inner join as long as 2 tables have the column name same. I would prefer using the primary key column.
Related
So I have three tables: companies, addresses and company_address.
For optimization reasons I need to copy city column from addresses table to companies table. Relation between companies and addresses is many to one (as many companies can occupy same address). They are connected through company_address table, consisting of address_id and company_id columns.
I found this solution for case without intermediate table: How to copy one column of a table into another table's column in PostgreSQL comparing same ID
Trying to modify query I came up with:
UPDATE company SET company.city=foo.city
FROM (
SELECT company_address.company_id, company_address.address_id, address.city
FROM address LEFT JOIN company_address
ON address.id=company_address.address_id
) foo
WHERE company.id=foo.company_id;
but it gives error:
ERROR: column "company" of relation "company" does not exist
I cant figure out what is going on. I'll be grateful for any ideas.
You don't need a subquery for that. Also, refer in the SET clause to your table columns without preceding with table name.
I believe that since your WHERE condition includes joined table, it should be INNER JOIN instead of a LEFT JOIN.
UPDATE company c
SET city = a.city
FROM address a
INNER JOIN company_address ca ON a.id = ca.address_id
WHERE c.id = ca.company_id
Note how using aliases for table names shortens the code and makes it readable at the very first glance.
You're right syntactically, you just don't need the table name at the beginning of the update statement:
UPDATE company SET city=foo.city
FROM (
SELECT company_address.company_id, company_address.address_id, address.city
FROM address LEFT JOIN company_address
ON address.id=company_address.address_id
) foo
WHERE company.id=foo.company_id;
In MS Access, I am trying to implement a many-to-many table that will store 2-way relationships, similar to Association between two entries in SQL table. This table stores info such as "Person A and Person B are coworkers" "C and D are friends", etc. The table is like this:
ConstitRelationships
LeftId (number, primary key, foreign key to Constituents.ConstitId)
RightId (number, primary key, foreign key to Constituents.ConstitId)
Description (text)
Note that the primary key is a composite of the two Id fields.
Also the table has constraints:
[LeftId]<>[RightId] AND [LeftId]<[RightId]
The table is working ok in my Access project, except that I cannot figure out how to make an updateable query that I want to use as a datasheet subform so users can easily add/delete records and change the descriptions. I currently have a non-updatable query:
SELECT Constituents.ConstituentId, Constituents.FirstName,
Constituents.MiddleName, Constituents.LastName,
ConstitRelationships.Description, ConstitRelationships.LeftId,
ConstitRelationships.RightId
FROM ConstitRelationships INNER JOIN Constituents ON
(Constituents.ConstituentId =
ConstitRelationships.RightId) OR (Constituents.ConstituentId =
ConstitRelationships.LeftId);
If I ignore the possibility that the constituentId I want is in the leftId column, I can do this, which is updatable. So the OR condition in the inner join above is what's messing it up.
SELECT Constituents.ConstituentId, Constituents.FirstName,
Constituents.MiddleName, Constituents.LastName,
ConstitRelationships.Description, ConstitRelationships.LeftId,
ConstitRelationships.RightId
FROM ConstitRelationships INNER JOIN Constituents ON
(Constituents.ConstituentId =
ConstitRelationships.RightId) ;
I also tried this wacky iif thing to collapse the two LeftId and RightId fields into FriendId, but it was not updateable either.
SELECT Constituents.ConstituentId, Constituents.FirstName,
Constituents.MiddleName,
Constituents.LastName, subQ.Description
FROM Constituents
INNER JOIN (
SELECT Description, Iif([Forms]![Constituents Form]![ConstituentId] <>
ConstitRelationships.LeftId, ConstitRelationships.LeftId,
ConstitRelationships.RightId) AS FriendId
FROM ConstitRelationships
WHERE ([Forms]![Constituents Form]![ConstituentId] =
ConstitRelationships.RightId)
OR ([Forms]![Constituents Form]![ConstituentId] =
ConstitRelationships.LeftId)
) subQ
ON (subQ.FriendId = Constituents.ConstituentId)
;
How can I make an updatable query on ConstitRelationships, including a JOIN with the Constituent.FirstName MiddleName LastName fields?
I am afraid that is not possible. Because you use joins in your query over three tables it is not updatable. There is no way around this.
Here some detailed information about the topic: http://www.fmsinc.com/Microsoftaccess/query/non-updateable/index.html
As mentioned in the linked article one possible solution and in my opinion best solution for you would be the temporary table. It is a load of work compared to the easy "bind-form-to-a-query"-approach but it works best.
The alternative would be to alter your datascheme in that way that you do not need joins. But then denormalized data and duplicates would go rampage which makes the temporary table a favorable choice.
So I have two tables, disciplinary and employees. Disciplinary has a column that lists an employee ID (an investigator) and an attempt to import new columns that are drawn from the employee table that yield the employee first and last name based on the existing employee ID from the disciplinary table. Below is the SQL I have so far:
SELECT d.*
, inv.firstName as investigatorFirstName
, inv.lastName as investigatorLastName
FROM det_siu_disciplinary d
LEFT OUTER JOIN cpso_employees inv ON
inv.commissionNumber = d.investigatorEmployeeID
WHERE d.isDelete = 0
This statement successfully adds the joined columns with their new names, but all columns are null. My primary concern is my SQL being flat out wrong, as it's the part of this process that I have least experience with. These statements are part of a much larger query, so if at all possible I'd prefer to not write a new query...adding contingencies would be perfect!
Anyone that assists, thank you in advance :)
the primary key column "CommissionNumber" seems unlikely to me to be the primary key of a table that should contain "EmployeeID" values, in order to join to the foreign key columns of your d table.
I'm designing a table that will hold numeric values for 2-3 situations of data:
Situation 1: has Age and Sex, along with the numeric value
Situation 2: has only Age, along with the numeric value
Situation 3: has only Sex, along with the numeric value
I don't want to create 3 different tables. Instead, only one table, with the following columns:
AgeID (references a table that contains information about the Age)
SexID (references a table that contains information about the Age)
Value (the numeric value itself)
AgeID and SexID as Foreign Keys and linked to the appropriate tables.
The problem is: my query is always doing a INNER JOIN with Age and Sex tables. For Situation 1 it works well because values are present. For Situations 2 and 3 I don't get any data, because either AgeID or SexID is null.
What solution is the correct one?
Change something in the table design?
Use Entity-Attribute-Value table to be more generic?
Use LEFT JOIN instead of INNER JOIN for all queries involving the nullable columns??
Any other idea?
Could someone clarify?
Thanks!
Yes an outer Join, Left or right, the Inner join is meant to filter out everything that doesn't have a match in both tables.
Use a conditional INNER JOIN, like
INNER JOIN Table x ON
(AgeID IS NULL OR AgeID = x.AgeID)
AND (SexID IS NULL OR SexID = x.SexID)
I have got a problem in selecting the employees name from employees table(empID) which is primary key and from consignation table based on two foreign key 1- consignee and 2- hand over by ( these two fields have relation with empID) so is it possible to select consignee and handoverby not by ID but based on Employees name(emp.name). Please write the query, thanks in adv.
Basically, you can use INNER JOIN if both column from consignation table are non-nullable. But if one is nullable, you need to use LEFT JOIN so records from consignation will still be shown on the list.
SELECT con.EmpName AS consigneeName,
ho.EmpName AS handOverName
FROM consignation a
INNER JOIN employee con
ON a.ConSignee = con.empID
INNER JOIN employee ho
ON a.handoverby = ho.empID
To further gain more knowledge about joins, kindly visit the link below:
Visual Representation of SQL Joins