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.
Related
I am learning sql now and practicing the scenarios to design the tables. I have a one scenario where I could not find proper suitable table structure.
The scenarios is as follows, I want to store depedencies user journey in sql. For example, in customer creation journey, we need to create valid sector, language and country codes in the system. Another example, to create a new account (bank account), we need to create the sector, language and country followed by customer.
So far, I could think of following table design, but I am sure this is not good as there is no primary key and not following the normalization standards.
journey
dependent
order
CUSTOMER
SECTOR
0
CUSTOMER
LANGUAGE
1
CUSTOMER
COUNTRY
2
ACCOUNT
CUSTOMER
0
I understand that this is many to many relationship as one journey can have many dependent and one dependent can be associated with multiple journeys. I need help to efficiently design the tables in sql, please can anyone help on this.
You need from the intermediate/join table that should look like this -
Table name - journey_dependent
Coll(Jurney_FK) Coll(Dependent_FK)
journey_id dependent_id
You can check more here - https://www.baeldung.com/jpa-many-to-many#1-modeling-a-many-to-many-relationship
If journey and dependent values are PK in origin tables, you have 2 FK. You can create a composite PK on that table with that 2 columns.
Maybe order need to be in dependent table. If not, there is information on that table : order. So this is not a pur relationship table. So you could optionally had a technical PK column (auto increment) on it.
I have a table joined from two other tables. I would like this table to stay updated with entries in the other two tables.
First Table is "employees"
I am using the ID, Last_Name, and First_Name.
And the second Table is "EmployeeTimeCardActions"
using columns ID, ActionTime, ActionDate, ShiftStart, and ActionType.
ID is my common column that the join was created by..Joined Table...
Because I usually have a comment saying I did not include enough information, I do not need a exact specific code sample and I think I have included everything needed. If there is a good reason to include more I will, I just try to keep as little company information public as possible
Sounds like you're having your data duplicated across tables. Not a smart idea at all. You can update data in one table when a row is updated in a different one via triggers but this is a TERRIBLE approach. If you want to display data joined from 2 tables, the right approach here is using an SQL VIEW which will display the current data.
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 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;
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