So I am a first time user here, and still relatively new to SQL. I am attempting to take 2 tables, and join them in a sense.
In Table1 I have the data:
House_Key ---Other Fields--- Customer_ID
House_Key is not unique, and as such I can have multiple Customer_IDs per household. House_Key is a numerical code, and Customer_ID is either Home, Business, or Bulk.
In Table2 I have the House_Key field, but not the Customer_ID field.
My goal is to have a new table that holds the fields of Table2 with a field called Customer_ID, but instead of having a new row for each type of Customer_ID like in Table 1, I want to have each House_Key only have one row, and the Customer_ID say something like "Home Business Bulk" if it is all three or any combination of them, but would prefer that it always have Home before Business before Bulk in the field.
Is there any way to do this? Thank you very much ahead of time.
Also, not sure if it matters, but in case it does I'm using SQL Server 2005.
Wow, I'm glad that you came here for an answer, but seems that you really need some reading about relational databases.
Instead of writing looong answer here're the links:
http://en.wikipedia.org/wiki/Database_normalization
http://en.wikipedia.org/wiki/Join_(SQL)
Related
SELECT DISTINCT
...
...
...
FROM Reviews Rev
INNER JOIN Reviews SubRev ON Subrev.W_ID=Rev.ID
WHERE Rev.Status='Approved'
This is a small part of a long query that I've been trying to understand for a day now. What is happening with the join? Reviews table appears to be joined with itself, under different aliases. Why is this done? What does it achieve? Also, ID field of the Reviews table is null for the entries that are nevertheless selected and returned. This is correct, but I don't understand how that can happen if the W_ID field is not null.
It allows you to join one row from the table to a different row in the table.
I've both seen this done, and used it myself, in cases where you maybe have a relationship between those rows.
Real-world examples:
An old version of a record and a newer version
Some sort of hierarchical relationship (e.g. if the table contains records of people, you can record that someone is a parent of someone else). There are probably plenty of other possible use cases, too.
SQL allows you to create a foreign key which relates between two different columns in the same table.
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 a table where I have these fields:
id(primary key, auto increment)
car registration number
car model
garage id
and 31 fields for each day of the mont for each row.
In these fields I have char of 1 or 2 characters representing car status on that date. I need to make a query to get number of each possibility for that day, field of any day could have values: D, I, R, TA, RZ, BV and LR.
I need to count in each row, amount of each value in that row.
Like how many I , how many D and so on. And this for every row in table.
What best approach would be here? Also maybe there is better way then having field in database table for each day because it makes over 30 fields obviously.
There is a better way. You should structure the data so you have another table, with rows such as:
CarId
Date
Status
Then your query would simply be:
select status, count(*)
from CarStatuses
where date >= #month_start and date < month_end
group by status;
For your data model, this is much harder to deal with. You can do something like this:
select status, count(*)
from ((select status_01 as status
from t
) union all
(select status_02
from t
) union all
. . .
(select status_31
from t
)
) s
group by status;
You seem to have to start with most basic tutorials about relational databases and SQL design. Some classic works like "Martin Gruber - Understanding SQL" may help. Or others. ATM you miss the basics.
Few hints.
Documents that you print for user or receive from user do not represent your internal data structures. They are created/parsed for that very purpose machine-to-human interface. Inside your program should structure the data for easy of storing/processing.
You have to add a "dictionary table" for the statuses.
ID / abbreviation / human-readable description
You may have a "business rule" that from "R" status you can transition to either "D" status or to "BV" status, but not to any other. In other words you better draft the possible status transitions "directed graph". You would keep it in extra columns of that dictionary table or in one more specialized helper table. Dictionary of transitions for the dictionary of possible statuses.
Your paper blank combines in the same row both totals and per-day detailisation. That is easy for human to look upon, but for computer that in a sense violates single responsibility principle. Row should either be responsible for primary record or for derived total calculation. You better have two tables - one for primary day by day records and another for per-month total summing up.
Bonus point would be that when you would change values in the primary data table you may ask server to automatically recalculate the corresponding month totals. Read about SQL triggers.
Also your triggers may check if the new state properly transits from the previous day state, as described in the "business rules". They would also maybe have to check there is not gaps between day. If there is a record for "march 03" and there is inserted a new the record for "march 05" then a record for "march 04" should exists, or the server would prohibit adding such a row. Well, maybe not, that is dependent upon you business processes. The general idea is that server should reject storing any data that is not valid and server can know it.
you per-date and per-month tables should have proper UNIQUE CONSTRAINTs prohibiting entering duplicate rows. It also means the former should have DATE-type column and the latter should either have month and year INTEGER-type columns or have a DATE-type column with the day part in it always being "1" - you would want a CHECK CONSTRAINT for it.
If your company has some registry of cars (and probably it does, it is not looking like those car were driven in by random one-time customers driving by) you have to introduce a dictionary table of cars. Integer ID (PK), registration plate, engine factory number, vagon factory number, colour and whatever else.
The per-month totals table would not have many columns per every status. It would instead have a special row for every status! The structure would probably be like that: Month / Year / ID of car in the registry / ID of status in the dictionary / count. All columns would be integer type (some may be SmallInt or BigInt, but that is minor nuancing). All the columns together (without count column) should constitute a UNIQUE CONSTRAINT or even better a "compound" Primary Key. Adding a special dedicated PK column here in the totaling table seems redundant to me.
Consequently, your per-day and per-month tables would not have literal (textual and immediate) data for status and car id. Instead they would have integer IDs referencing proper records in the corresponding cars dictionary and status dictionary tables. That you would code as FOREIGN KEY.
Remember the rule of thumb: it is easy to add/delete a row to any table but quite hard to add/delete a column.
With design like yours, column-oriented, what would happen if next year the boss would introduce some more statuses? you would have to redesign the table, the program in many points and so on.
With the rows-oriented design you would just have to add one row in the statuses dictionary and maybe few rows to transition rules dictionary, and the rest works without any change.
That way you would not
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 am trying to develop a SQL Server 2005 query but I'm being unsuccessful at the moment. I trying every different approach that I know, like derived tables, sub-queries, CTE's, etc, but I couldn't solve the problem. I won't post the queries I tried here because they involve many other columns and tables, but I will try to explain the problem with a simpler example:
There are two tables: PARTS_SOLD and PARTS_PURCHASED. The first contains products that were sold to customers, and the second contains products that were purchased from suppliers. Both tables contains a foreign key associated with the movement itself, that contains the dates, etc.
Here is the simplified schema:
Table PARTS_SOLD:
part_id
date
other columns
Table PARTS_PURCHASED
part_id
date
other columns
What I need is to join every row in PARTS_SOLD with a unique row from PARTS_PURCHASED, chose by part_id and the maximum "date", where the "date" is equal of before the "date" column from PARTS_PURCHASED. In other words, I need to collect some information from the last purchase event for the item for every event of selling this item.
The problem itself is that I didn't find a way of joining the PARTS_PURCHASED table with PARTS_SOLD table using the column "date" from PARTS_SOLD to limit the MAX(date) of the PARTS_PURCHASED table.
I could have done this with a cursor to solve the problem with the tools I know, but every table has millions of rows, and perhaps using cursors or sub-queries that evaluate a query for every row would make the process very slow.
You aren't going to like my answer. Your database is designed incorrectly which is why you can't get the data back out the way you want. Even using a cursor, you would not get good data from this. Assume that you purchased 5 of part 1 on May 31, 2010. Assume on June 1, you sold ten of part 1. Matching just on date, you would match all ten to the May 31 purchase even though that is clearly not correct, some parts might have been purchased on May 23 and some may have been purchased on July 19, 2008.
If you want to know which purchased part relates to which sold part, your database design should include the PartPurchasedID as part of the PartsSold record and this should be populated at the time of the purchase, not later for reporting when you have 1,000,000 records to sort through.
Perhaps the following would help:
SELECT S.*
FROM PARTS_SOLD S
INNER JOIN (SELECT PART_ID, MAX(DATE)
FROM PARTS_PURCHASED
GROUP BY PART_ID) D
ON (D.PART_ID = S.PART_ID)
WHERE D.DATE <= S.DATE
Share and enjoy.
I'll toss this out there, but it's likely to contain all kinds of mistakes... both because I'm not sure I understand your question and because my SQL is... weak at best. That being said, my thought would be to try something like:
SELECT * FROM PARTS_SOLD
INNER JOIN (SELECT part_id, max(date) AS max_date
FROM PARTS_PURCHASED
GROUP BY part_id) AS subtable
ON PARTS_SOLD.part_id = subtable.part_id
AND PARTS_SOLD.date < subtable.max_date