I'm working on a football statistics database, and in the table to store results of matches, I have two references to the primary key of a team table: one home, one away.
My intention is to create a query which returns the name of both of the teams, along with other details, but I can't think of a way to achieve this WITH the team names (my attempts so far can only produce one team name, with the other an ID number). I'll give the relation structure if this wasn't clear:
(PKs in bold, FKs asterisk)
team(team_id, team_name, venue)
match(match_id, home_team*, away_team*, home_score, away_score, date,)
My desired output would be a table with these columns:
home_team_name, home_team_score, away_team_score, away_team_name, date, venue
Is this possible with my tables, or should I change the way I store results?
When joining the team table to the match table in a query, you'll need to join the match table to the team table twice. You need to use an different alias for the teams each time.
Related
Let's assume I am building the backend of a university management software.
I have a users table with the following columns:
id
name
birthday
last_english_grade
last_it_grade
profs table columns:
id
name
birthday
I'd like to have a third table with which I can determine all professors teaching a student.
So I'd like to assign multiple teachers to each student.
Those Professors may change any time.
New students may be added any time too.
What's the best way to achieve this?
The canonical way to do this would be to introduce a third junction table, which exists mainly to relate users to professors:
users_profs (
user_id,
prof_id,
PRIMARY KEY (user_id, prof_id)
)
The primary key of this junction table is the combination of a user and professor ID. Note that this table is fairly lean, and avoids the problem of repeating metadata for a given user or professor. Rather, user/professor information remains in your two original tables, and does not get repeated.
I had a very big excel spreadsheet that I moved into Access to try to deal with it easier. I'm very much a novice. I'm trying to use SQL via Access.
I need to assign a unique identifier to duplicates. I've seen people use DENSE_RANK in SQL but I can't get it to work in Access.
Here's what I'm trying to do: I have a large amount of patient and sample data (20k rows). My columns are called FULL_NAME, SAMPLE_NUM, and DATE_REC. Some patients have come in more than once and have multiple samples. I want to give each patient a unique ID that I want to call PATIENT_ID.
I can't figure out how to do this, aside from typing it out on each row. I would greatly appreciate help as I really don't know what I'm doing and there is no one at my work who can help.
To illustrate the previous answers' textual explanation, consider the following SQL action queries which can be run in an Access query window one by one or as VBA string queries with DAO's CurrentDb.Execute or DoCmd.RunSQL. The ALTER statements can be done in MSAcecss.exe.
Create a Patients table (make-table query)
SELECT DISTINCT s.FULL_NAME INTO myPatientsTable
FROM mySamplesTable s
WHERE s.FULL_NAME IS NOT NULL;
Add an autonumber field to new Patients table as a Primary Key
ALTER TABLE myPatientsTable ADD COLUMN PATIENT_ID AUTOINCREMENT NOT NULL PRIMARY KEY;
Add a blank Patient_ID column to Samples table
ALTER TABLE mySamplesTable ADD COLUMN PATIENT_ID INTEGER;
Update Patient_ID Column in Samples table using FULL_NAME field
UPDATE mySamplesTable s
INNER JOIN myPatientsTable p
ON s.[FULL_NAME] = p.[FULL_NAME]
SET s.PATIENT_ID = p.PATIENT_ID;
Maintain third-norm principles of relational databases and remove FULL_NAME field from Samples table
ALTER TABLE mySamplesTable DROP COLUMN FULL_NAME;
Then in a separate query, add a foreign key constraint on PATIENT_ID
ALTER TABLE mySamplesTable
ADD CONSTRAINT PatientRelationship
FOREIGN KEY (PATIENT_ID)
REFERENCES myPatientsTable (PATIENT_ID);
Sounds like FULL_NAME is currently the unique identifier. However, names make very poor unique identifiers and name parts should be in separate fields. Are you sure you don't have multiple patients with same name, e.g. John Smith?
You need a PatientInfo table and then the SampleData table. Do a query that pulls DISTINCT patient info (apparently this is only one field - FULL_NAME) and create a table that generates unique ID with autonumber field. Then build a query that joins tables on the two FULL_Name fields and updates a new field in SampleData called PatientID. Delete the FULL_Name field from SampleData.
The command to number rows in your table is [1]
ALTER TABLE MyTable ADD COLUMN ID AUTOINCREMENT;
Anyway as June7 pointed out it might not be a good idea to combine records just based on patient name as there might be duplicates. Better way will be treat each record as unique patient for now and have a way to fix patient ID when patient comes back. I would suggest to go this way:
create two new columns in your samples table
ID with autoincrement as per query above
patientID where you will copy values from ID column - for now they will be same. But in future they will diverge
copy columns patientID and patientName into separate table patients
now you can delete patientName column from samples table
add column imported to patients table to indicate, that there might be some other records that belong to this patient.
when patients come back you open his record, update all other info like address, phone, ... and look for all possible samples record that belong to him. If so, then fix patient id in those records.
Now you can switch imported indicator because this patient data are up to date.
After fixing patientID for samples records. You will end up with patients with no record in samples table. So you can go and delete them.
Unless you already have a natural key you will be corrupting this data when you run the distinct query and build a key from it. From your posting I would guess a natural key would be SAMPLE_NUM. Another problem is that if you roll up by last name you will almost certainly be combining different patients into one.
I have an existing table which is expected to work for a new piece of functionality. I have the opinion that a new table is needed to achieve the objective and would like an opinion if it can work as is, or is the new table a must? The issue is a query returning more records than it should, I believe this is why:
There is a table called postcodes. Over time this has really become a town table because different town names have been entered so it has multiple records for most postcodes. In reference to the query below the relevant fields in the postcode table are:
postcode.postcode - the actual postcode, as mentioned this is not unique
postcode.twcid - is a foreign key to the forecast table, this is not unique either
The relevant fields in the forecast table are:
forecast.twcid - identifyer for the table however not unique because there four days worth of forecasts in the table. Only ever four, newver more, never less.
And here is the query:
select * from forecast
LEFT OUTER JOIN postcodes ON forecast.TWCID = postcodes.TWCID
WHERE postcodes.postcode = 3123
order by forecast.twcid, forecast.theDate;
Because there are two records in the postcode table for 3123 the results are doubled up. Two forecasts for day 1, two for day 2 etc......
Given that the relationship between postcodes and forecast is many to many (there are multiple records in the postcode tables for each postcode and twcid. And there are multiple records for each twcid in the forecast table because it always holds four days worth of forecasts) is there a way to re-write the query to only get four forecast records for a post code?
Or is my thought of creating a new postcode table which has unique records for each post code necessary?
You have a problem that postcodes can be in multiple towns. And towns can have multiple postcodes. In the United States, the US Census Bureau and the US Post Office have defined very extensive geographies for various coding schemes. The basic idea is that a zip code has a "main" town.
I would suggest that you either create a separate table with one row per postcode and the main town. Or, add a field to your database indicating a main town. You can guarantee uniqueness of this field with a filtered index:
create unique index postcode_postcode_maintown on postcodes(postcode) where IsMainTown = 1;
You might need the same thing for IsMainPostcode.
(Filtered indexes are a very nice feature in SQL Server.)
With this construct, you can change your query to:
select *
from forecast LEFT OUTER JOIN
postcodes
ON forecast.TWCID = postcodes.TWCID and postcodes.IsMainPostcode = 1
WHERE postcodes.postcode = 3123
order by forecast.twcid, forecast.theDate;
You should really never have a table without a primary key. Primary keys are, by definition, unique. The primary key should be the target for your foreign keys.
You're having problems because you're fighting against a poor database design.
I have a table students with the following fields.
Id,FirstName,SecondName,Photo,Student_ID
I also have a table called class_of_2011 with the fields
Id,FirstName,SecondName,Photo,Student_ID,Subjects
I want to select some specific students from table students and have them in table class_of_2011,but I already have the names in table students.I am thinking the only way to do this is to copy the names i want to the table class_of_2011,but since there will be a class of 2012 and beyond,I feel like I will be simply copying data from one table to the other.
Is repeating myself inevitable in my case?
It looks like this could be normalized easily. Why not have your class_of_ tables simply have a foreign key to the student table's id column?
StudentId,Subjects
In this way, one student record could be associated with several classes, in case someone is on the 5-year plan.
I'm assuming that the Student_ID field in the Students table is their id number or something, and not the primary key of that table.
Students Table
Id,FirstName,SecondName,Photo,Student_ID
Subjects Table
Id,Subject
Student_Subjects Table
Id,Student_Id,Subject_Id,Year
You may then assign a student multiple subjects, for multiple years.
The class_of_2011 table should contain the primary key of the students table and none of the other "repeated" data. All of the other columns you're interested in can then be obtained by joining the two columns together in a query.
I would restructure the data if possible... Something like....
Student Table
ID, Name, Address, other common specific to student
GraduatingClass Table
YearGraduate, StudentID
Enrollment Table
StudentID, ClassID, SemesterID
I have some tables that benefit from many-to-many tables. For example the team table.
Team member can hold more than one 'position' in the team, all the positions are listed in the position db table. The previous positions held are also stored for this I have a separate table, so I have
member table (containing team details)
positions table (containing positions)
member_to_positions table (id of member and id of position)
member_to_previous_positions (id of member and id of position)
Simple, however the crux comes now that a team member can belong to many teams aghhh.
I already have a team_to_member look-up table.
Now the problem comes how do I tie a position to a team? A member may have been team leader on one team, and is currently team radio man and press officer on a different team. How do I just pull the info per member to show his current position, but also his past history including past teams.
Do I need to add a position_to team table and somehow cross reference that, or can I add the team to the member to positions table?
It's all very confusing, this normalization.
Yes, a many-to-many junction table can have additional attributes (columns).
For example, if there's a table called PassengerFlight table that's keyed by PassengerID and FlightID, there could be a third column showing the status of the given passenger on the given flight. Two different statuses might be "confirmed" and "wait listed", each of them coded somehow.
In addition, there can be ternary relationships, relationships that involve three entities and not just two. These tables are going to have three foreign keys that taken together are the primary key for the relationship table.
It's perfectly legitimate to have a TeamPositionMember table, with the columns
Team_Id
Position_Code
Member_Id
Start_Date
End_Date NULLABLE
And and a surrogate ID column for Primary Key if you want; otherwise it's a 3-field composite Primary Key. (You'll want a uniqueness constraint on this anyway.)
With this arrangement, you can have a team with any set of positions. A team can have zero or more persons per position. A person can fill zero or more positions for zero or more teams.
EDIT:
If you want dates, just revise as shown above, and add Start_Date to the PK to allow the same person to hold the same position at different times.
My first thought:
Give your many-to-many teams/members table an ID column. Every team-to-member relationship now has an ID.
Then create a many-to-many linking positions to team-member relationships.
This way, teams can have multiple members, members can have multiple teams, and members can have multiple positions on a per-team basis.
Now everything is nice and DRY, and all the linking up seems to work. Does that sound right to anyone else?
Sounds like you need a many-to-many positions to teams table now.
Your team_to_member table can indeed have an extra column position_id to describe (or in this case point to) the position the member has within that team.
Get rid of member_to_previous_position table. Just use member_to_positions and have these columns:
MemberToPositionID (autoincrement OK only)
MemberID
PositionID
StartDate
EndDate
Then to find current positions, you do:
select *
from member_to_positions
where EndDate is null