I am creating queries for my database, and I want to know if the Artist_Name Field Value (A string) is part of a field in a different table, Band. I want to see if the Artist_Name is in the Band_Members field. Both are short text fields. I am using the Design View to create this query, which I will later edit in SQL. I will attach an Image to show my Query Relationships and Fields: Query Relationships
You can use DCount function to count the number of records in Band having Band_Members equal to Artist_Name.
You can create a left join from Artist table to Band table, on fields Artist_Name and Band_Members, but note that it may blow up the number of records in your output if artists are part of multiple bands.
I don't understand the field Band.Band_Members though; I suppose there is some one-to-may relationship between a band and it's members.
Related
I got my query in MS Access duplicating its errors. The data is being entered from a Windows Forms app, and the data is in two separate tables. I'm using a basic query to show each row in the table. Name table and score table.
Now the issue is that the data is duplicating itself.
Image of the access table
This is the SQL query:
SELECT DISTINCT score.score, name.Name
FROM name, score;
Table Schema
What SQL code could I use to stop the duplication error?
Thanks in advance
Need to add a long integer number field StudID in Scores table then do data entry to add the appropriate student identifier value into each Scores record. This will establish a primary/foreign key relationship between tables that can be used in query with a LEFT or RIGHT JOIN on key fields.
This assumes a student can have multiple scores (for what?). If there can be multiple scores, then probably also need a date field ScoreDate and then possibly should be a Subject field.
If student can have only 1 score, then don't need Scores table - have a field Score in Students table.
Strongly advise not to use reserved words as names for anything. Name is a reserved word.
I'm having an exercise requiring to create two table for a travel business:
Activity
Booking
it turns out that the column activities in the Booking table references from the Activities table. However it contains multiple value. How do I sort it out? If I insert multiple rows there will possibly duplication in the Booking's primary key.
As Gordon mentioned you should refactor your tables for better normalization. If I interpret your intent correctly this is more like what your schema should look like. Booking should only contain an ID for adventure and an ID for Customer. You will add a row to [AdventureActivity] for each activity booked on a [Booking]. With this design you can JOIN tables and get all the data you require without having to try to parse out multiple values in a column.
I may be total standard here, but I have a table with duplicate values across the records i.e. People and HairColour. What I need to do is create another table which contains all the distinct HairColour values in the Group of Person records.
i.e.
Name HairColour
--------------------
Sam Ginger
Julie Brown
Peter Brown
Caroline Blond
Andrew Blond
My Person feature view needs to list out the distinct HairColours:
HairColour Ginger
HairColour Brown
HairColour Blond
Against each of these Person feature rows I record the Recommended Products.
It is a bit weird from a Relational perspective, but there are reasons. I could build up the Person Feature"View as I add Person records using say an INSTEAD OF INSERT trigger on the View. But it gets messy. An alternative is just to have Person Feature as a View based on a SELECT DISTINCT of the Person table and then link Recommended Products to this. But I have no Primary Key on the Person Feature View since it is a SELECT DISTINCT View. I will not be updating this View. Also one would need to think about how to deal with the Person Recommendation records when a Person Feature record disappeared since since it is not based on a physical table.
Any thoughts on this please?
Edit
I have a table of People with duplicate values for HairColour across a number of records, e.g., more than one person has blond hair. I need to create a table or view that represents a distinct list of "HairColour" records as above. Against each of these "HairColour" records I need link another table called Product Recommendation. The main issue to start with is creating this distinct list of records. Should it be a table or could it be a View based on a SELECT DISTINCT query?
So Person >- HairColour (distinct Table or Distinct View) -< Product Recommendation.
If HairColour needs to be a table then I need to make sure it has the correct records in it every time a Person record is added. Obviously using a View would do this automatically, but I am unsure whether you can can hang another table off a View.
If I understand correctly, you need a table with a primary key that lists the distinct hair colors that are found in a different table.
CREATE TABLE Haircolour(
ID INT IDENTITY(1,1) NOT NULL,
Colour VARCHAR(50) NULL
CONSTRAINT [PK_Haircolour] PRIMARY KEY CLUSTERED (ID ASC))
Then insert your records. If this is querying a table called "Person" it will look like this:
INSERT INTO Haircolour (Colour) SELECT DISTINCT HairColour FROM Person
Does this do what you are looking for?
UPDATE:
Your most recent Edit shows that you are looking for a many-to-many relationship between the Person and ProductRecommendation tables, with the HairColour table functioning as a cross reference table.
As ErikE points out, this is a good opportunity to normalize your data.
Create the HairColour table as described above.
Populate it from whatever source you like, for example the insert statement above.
Modify both the Person and the ProductRecommendation tables to include a HairColourID field, which is an integer foreign key that points to the PK field of the HairColour table.
Update Person.HairColourID to point to the color mentioned in the Person.HairColour column.
Drop the Person.HairColour column.
This involves giving up the ability to put free form new color names into the Person table. Any new colors must now be added to the HairColour table; those are the only colors that are available.
The foreign key constraint enforces the list of available colors. This is a good thing. Referential integrity keeps your data clean and prevents a lot of unexpected errors.
You can now confidently build your ProductRecommendation table on a data structure that will carry some weight.
Are you simply looking for a View of distinct hair colors?
CREATE VIEW YourViewName AS
SELECT DISTINCT HairColour
FROM YourTableName
You can query this view like a table:
SELECT 'HairColour: ' + HairColour
FROM YourViewName
If you are trying to create a new (temp) table, the syntax would look like:
SELECT Name, HairColour
INTO #Temp
FROM YourTableName
GROUP BY Name, HairColour
Here the GROUP BY is doing the same work that a DISTINCT keyword would do in the select list. This will create a temp table with unique combinations of "Name" and "HairColour".
You need to clear up a few things in your post (or in your mind) first:
1) What are the objectives? Forget about tables and views and whatever. Phrase your objectives as an ordinary person would. For example, from what I could gather from your post:
"My objective is to have a list of recommended products based on each person's hair colour."
2) Once you have that, check what data you have. I assume you have a "Persons" table, with the columns "Name" and "HairColour". You check your data and ask yourself: "Do I need any more data to reach my objective?" Based on your post I say yes: you also need a "matching" between hair colours and product ids. This must be provided, or programmed by you. There is no automatic method of saying for example "brown means products X,Y,Z.
3) After you have all the needed data, you can ask: Can I perform a query that will return a close approximation of my objective?
See for example this fiddle:
http://sqlfiddle.com/#!2/fda0d6/1
I have also defined your "Select distinct" view, but I fail to see where it will be used. Your objectives (as defined in your post) do not make this clear. If you provide a thorough list in Recommended_Products_HairColour you do not need a distinct view. The JOIN operation takes care of your "missing colors" (namely "Green" in my example)
4) When you have the query, you can follow up with: Do I need it in a different format? Is this a job for the query or the application? etc. But that's a different question I think.
When I have to select a number of fields from different tables:
do I always need to join tables?
which tables do I need to join?
which fields do I have to use for the join/s?
do the joins effects reflect on fields specified in select clause or on where conditions?
Thanks in advance.
Think about joins as a way of creating a new table (just for the purposes of running the query) with data from several different sources. Absent a specific example to work with, let's imagine we have a database of cars which includes these two tables:
CREATE TABLE car (plate_number CHAR(8),
state_code CHAR(2),
make VARCHAR(128),
model VARCHAR(128),);
CREATE TABLE state (state_code CHAR(2),
state_name VARCHAR(128));
If you wanted, say, to get a list of the license plates of all the Hondas in the database, that information is already contained in the car table. You can simply SELECT * FROM car WHERE make='Honda';
Similarly, if you wanted a list of all the states beginning with "A" you can SELECT * FROM state WHERE state_name LIKE 'A%';
In either case, since you already have a table with the information you want, there's no need for a join.
You may even want a list of cars with Colorado plates, but if you already know that "CO" is the state code for Colorado you can SELECT * FROM car WHERE state_code='CO'; Once again, the information you need is all in one place, so there is no need for a join.
But suppose you want a list of Hondas including the name of the state where they're registered. That information is not already contained within a table in your database. You will need to "create" one via a join:
car INNER JOIN state ON (car.state_code = state.state_code)
Note that I've said absolutely nothing about what we're SELECTing. That's a separate question entirely. We also haven't applied a WHERE clause limiting what rows are included in the results. That too is a separate question. The only thing we're addressing with the join is getting data from two tables together. We now, in effect, have a new table called car INNER JOIN state with each row from car joined to each row in state that has the same state_code.
Now, from this new "table" we can apply some conditions and select some specific fields:
SELECT plate_number, make, model, state_name
FROM car
INNER JOIN state ON (car.state_code = state.state_code)
WHERE make = 'Honda'
So, to answer your questions more directly, do you always need to join tables? Yes, if you intend to select data from both of them. You cannot select fields from car that are not in the car table. You must first join in the other tables you need.
Which tables do you need to join? Whichever tables contain the data you're interested in querying.
Which fields do you have to use? Whichever fields are relevant. In this case, the relationship between cars and states is through the state_code field in both table. I could just as easily have written
car INNER JOIN state ON (state.state_code = car.plate_number)
This would, for each car, show any states whose abbreviations happen to match the car's license plate number. This is, of course, nonsensical and likely to find no results, but as far as your database is concerned it's perfectly valid. Only you know that state_code is what's relevant.
And does the join affect SELECTed fields or WHERE conditions? Not really. You can still select whatever fields you want and you can still limit the results to whichever rows you want. There are two caveats.
First, if you have the same column name in both tables (e.g., state_code) you cannot select it without clarifying which table you want it from. In this case I might write SELECT car.state_code ...
Second, when you're using an INNER JOIN (or on many database engines just a JOIN), only rows where your join conditions are met will be returned. So in my nonsensical example of looking for a state code that matches a car's license plate, there probably won't be any states that match. No rows will be returned. So while you can still use the WHERE clause however you'd like, if you have an INNER JOIN your results may already be limited by that condition.
Very broad question, i would suggest doing some reading on it first but in summary:
1. joins can make life much easier and queries faster, in a nut shell try to
2. the ones with the data you are looking for
3. a field that is in both tables and generally is unique in at least one
4. yes, essentially you are createing one larger table with joins. if there are two fields with the same name, you will need to reference them by table name.columnname
do I always need to join tables?
No - you could perform multiple selects if you wished
which tables do I need to join?
Any that you want the data from (and need to be related to each other)
which fields do I have to use for the
join/s?
Any that are the same in any tables within the join (usually primary key)
do the joins effects reflect on fields specified in select clause or on where conditions?
No, however outerjoins can cause problems
(1) what else but tables would you want to join in mySQL?
(2) those from which you want to correlate and retrieve fields (=data)
(3) best use indexed fields (unique identifiers) to join as this is fast. e.g. join
retrieve user-email and all the users comments in a 2 table db
(with tables: tableA=user_settings, tableB=comments) and both having the column uid to indetify the user by
select * from user_settings as uset join comments as c on uset.uid = c.uid where uset.email = "test#stackoverflow.com";
(4) both...
I was wondering how can you display multiple values in a database for example, lets say you have a user who will fill out a form that asks them to type in what types of foods they like for example cookies, candy, apples, bread and so on.
How can I store it in the MySQL database under the same field called food?
How will the field food structure look like?
You may want to read the excellent Wikipedia article on database normalization.
You don't want to store multiple values in a single field. You want to do something like this:
form_responses
id
[whatever other fields your form has]
foods_liked
form_response_id
food_name
Where form_responses is the table containing things that are singular (like a person's name or address, or something where there aren't multiple values). foods_liked.form_response_id is a reference to the form_responses table, so the foods liked by the person who has response number six will have a value of six for the form_response_id field in foods_liked. You'll have one row in that table for each food liked by the person.
Edit: Others have suggested a three-table structure, which is certainly better if you are limiting your users to selecting foods from a predefined list. The three-table structure may be better in the case that you are allowing them the ability to enter their own foods, though if you go that route you'll want to be careful to normalize your input (trim whitespace, fix capitalization, etc.) so you don't end up with duplicate entries in that table.
normally, we do NOT work out like this. try to use a relation table.
Table 1: tbl_food
ID primary key, auto increment
FNAME varchar
Table 2: tbl_user
ID primary key, auto increment
USER varchar
Table 3: tbl_userfood
RID auto increment
USERID int
FOODID int
Use similar format to store your data, instead a chunk of data fitted into a field.
Querying in these tables are easier than parsing the chunk of data too.
Use normalization.
More specifically, create a table called users. Create another called foods. Then link the two tables together with a many-to-many table called users_to_foods referencing each others foreign keys.
One way to do it would be to serialize the food data in your programming language, and then store it in the food field. This would then allow you to query the database, get the serialized food data, and convert it back into a native data structure (probably an array in this case) in your programming language.
The problem with this approach is that you will be storing a lot of the same data over and over, e.g. if a lot of people like cookies, the string "cookies" will be stored over and over. Another problem is searching for everyone who likes one particular food. To do that, you would have to select the food data for each record, unserialize it, and see if the selected food is contained within. This is a very inefficient.
Instead you'll want to create 3 tables: a users table, a foods table, and a join table. The users and foods tables will contain one record for each user and food respectively. The join table will have two fields: user_id and food_id. For every food a user chooses as a favorite, it adds a record to the join table of the user's ID and the food ID.
As an example, to pull all the users who like a particular food with id FOOD_ID, your query would be:
SELECT users.id, users.name
FROM users, join_table
WHERE join_table.food_id = FOOD_ID
AND join_table.user_id = users.id;