I'm trying to join two tables in peewee with a mysql database. This is pretty easy doing something like this:
s = Table1.select(Table1, Table2).join(
Table2).naive().where(Table1.Title == "whatever")
Unfortunately, I have called a column in Table1 and Table2 the same thing, "URL". Then when I select s.URL it gives me the URL from Table2, which I don't want, I want the one from Table1. Is there some way to either not join the Table2.URL column or to name it something different? This question seems to be addressing a similar problem in regular SQL (not peewee), is there a way to do something similar in peewee?
In other words, I think I'm looking for either a "JOIN AS" method or a "DON'T JOIN THIS COLUMN" method in peewee.
Thanks a lot,
Alex
I haven't used peewee, but the docs suggest that any table, expression, or column has a .alias() method. But that means you'd have to alias the URL column individually:
Table1.select(Table1, Table2.URL.alias('t2_url')).join(Table2)...
Even in hand-crafted SQL, you can't SELECT Table2.* but simultaneously give an alias for one of the columns of Table2.
Related
It is a bad habbit to use select "*" when writing SQL with JDBC.
But what if there are many columns in the table(s), how can I easily pickup some wanted columns in the SQL rather than using *?
For example,suppose I have table1 and table2,which have 10 columns each. I need most of their columns, but I do not want to use SELECT * FROM table1 inner join table2 on table1.id=table2.id since this is a bad practice. Actually I only need SELECT column1,column4,column5(most of them)...... from table1 inner join table2 on table1.id=table2.id.
So, do I have a way that replace * into all columns and then I can just COPY this text and remove some columns which I don't need and remain what I want rather than type each column name?
If you're using SQL Server, an easy trick is to open a database in Management Studio, right-click on a table and select "Select top 1000 Rows" from the context menu. You'll get all the fields listed one by one. You can copy and paste the text wherever you want.
I don't know which IDE you're using let alone which DBMS, but there's probably a trick like this one in it.
Try using the query below which is use to get all the columns of a specific table. Hope it helps you.
SELECT name FROM sys.all_columns WHERE object_id = OBJECT_ID('MyTableHere')
In MySQL, try the code below or you can refer in this Documentation:
SHOW COLUMNS FROM MyTableNameHere;
I have a table, tblNoComp, that has two columns, both foreign keys pointing to tblPackage.ID. The purpose of tblNoComp is to store which packages are not compatible with each other, by simply storing the ID of those packages in two columns, OneID and TwoID.
May not be the best way of storing it, but since multiple packages aren't compatible with others, it seemed to be the most logical.
Attempting to create a view that shows the tblPackage.Name for the two side by side - I have the following, but unsure how to get the TwoID Package Name..
SELECT tblNoComp.OneID, tblPackages.Package,tblNoComp.TwoID,tblPackages.Package
FROM tblNoComp, tblPackages
WHERE (tblNoComp.OneID = tblPackages.PID)
Currently the second tblPackages.Package is simply showing OneID name, not TwoID.. Not sure how to resolve?
Thank you!
--Apologies if a simple question, I've searched for an hour but haven't quite been able to describe my problem correctly.
The code you have in your comment:
SELECT
tblNoComp.OneID,
tblPackages.Package AS OneIDPackageName,
tblNoComp.TwoID,
tblPackages.Package AS TwoIDPackageName
FROM
tblNoComp
LEFT JOIN tblPackages
ON tblNoComp.OneID = tblPackages.PID
Is aliasing the columns instead of the tables. The idea behind the aliasing is to JOIN the same table twice as two different tables, using two different aliases. You're only joining it once and trying to use it twice.
You probably intent something more like this:
SELECT
tblNoComp.OneID,
tblOnePackages.Package AS OneIDPackageName,
tblNoComp.TwoID,
tblTwoPackages.Package AS TwoIDPackageName
FROM
tblNoComp
LEFT JOIN tblPackages AS tblOnePackages
ON tblNoComp.OneID = tblOnePackages.PID
LEFT JOIN tblPackages AS tblTwoPackages
ON tblNoComp.TwoID = tblTwoPackages.PID
(Note that I don't have a MySQL syntax checker handy, so this may need to be tweaked in order to run properly.)
Note that the same table is joined twice on two different keys, and that each time it's given a different alias so that it can be referenced within the SELECT clause as two separate tables.
I noticed te other day I can joins in mysql just as easily by doing,
SELECT peeps, persons, friends FROM tablea JOIN tableb USING (id) WHERE id = ?
In stead of using,
SELECT a.peeps, a.persons, b.friends FROM tablea a JOIN tableb b USING (id) WHERE id = ?
It only works if there is no matching column names, why should I do the second rather than the first?
No, you don't need to, but in my humble opinion you really should. It's almost always better in my experience to be explicit with what you're trying to do.
Consider the feelings of the poor guy (or girl) who has to come behind you and try to figure out what you were trying to accomplish and in which tables each column resides. Explicitly stating the source of the column allows one to look at the query and glean that information without deep knowledge of the schema.
Query 1 will work (as long as there are no ambiguous column names).
Query 2 will
be clearer
be more maintainable (think of someone who doesn't know the database schema by heart)
survive the addition of an ambiguous column name to one of the tables
So, don't be lazy because of that pitiful few saved keystrokes.
It's not necessary if you have no duplicate column names. If you do, the query will fail.
I am trying to use the results of another query to use as a criteria for another. In my specific example, I might have four houses that are 'A', 'B', 'C', 'D' (the unique values of a field in a table called Homes).
I want to go through another query and say for each house type, what percent of residents (in Residents table) are married, which I want to do by using Count() to count the number for each Home type.
Do I need to loop through the results using VBA? Asking on a higher level, is there a way to use the results from a query as inputs into another - more than just limit the results of the new query to the results of the prior query?
Edit:
In semi-pseudo code:
For each (result of previous query) Do
New query WHERE field1 = (row of previous query)
End Do
What I am trying to ask, is there a way to accomplish this in Access using SQL? Or is this something that has to be done in VBA?
I know that if it can be done in SQL that would be the best performing and best practice, but I'm relatively inexperienced in SQL and online resources aren't always helpful because Access has it's own particular flavor of SQL.
Since you are using VBA to run this, you can loop through your recordsets and yes you can use a value from one query in the next query. There are alot of resources out there to help.
VBA: Working with RecordSets
Looping through Record Sets
Code through all records
To answer your general question, yes there is. You can do a nested query i.e. select column a from table a where column a = (select column b from table b where column b=x)
You can go as many levels deep as you want, but the caveat is the nested query can only return one column and with a specific answer set. You can also use select statements as your columns i.e
select (select column b from table b) col b from table a ..... Not the exact syntax but I would have to dig out some examples from an old project to find that.
Nested queries are useful, but for the level of precision you are looking for, a stored procedure or a view is probably a better option. Just for ease of use, I would look at creating a view of the data that you want and then querying from that to start with. More flexible than a nested query.
You need to join two tables using a common column and then get your specific column from any of the table
SELECT A.REQUIRED_FIELD from TABLEA AS A
INNER JOIN TABLEB AS B ON A.FOREIGN_KEY=B.FOREIGN_KEY
WHERE CONDITION
I'm thinking about it, which of the following two query types would give me the fastest performance for a user messaging module inside my site:
The first one i thought about is a multi table setup, which has a connection table, and a main table. The connection table holds the connection between accounts, and the messaging table.
In this case a query would look like following, to get some data of the author, and the messages he has sent:
SELECT m.*, a.username
FROM messages AS m
LEFT JOIN connection_table
ON (message_id = m.id)
LEFT JOIN accounts AS a
ON (account_id = a.id)
WHERE m.id = '32341'
Inserting into it is a little bit more "complicated".
My other idea, and in my thought the better solution of this problem is that i store the data i would use in a connection table in the same table where is store the data of the mail. Sounds like i would get lots of duplicated entries, but no, because i have a field which has text type and holds user ids like this: *24*32*249*
If I want to query them, i use the mysql LIKE method. Deleting is an other problem, but for this i have one more field where i store who has deleted the post.
Sad about that i don't know how to join this.
So what would you recommend? Are there other ways?
Sounds like you are using an n:m relation.. if yes, don't put a list of ids in a single column but create a mapping table containing two columns - the primary key of table1 and the primary key of table2. Then selecting, inserting and deleting will all be easy and still cheap.
I wonder how many messages will be send to multiple recipients? It might just be easier to have it all in one table - MessageID, SentFrom, SentTo, Message, and dup it for multiple people. This obviously makes it extremely easy to query.
Definately avoid storing multiple ID's in one field and using LIKE - that'll be a performance killer - go with ThiefMasters suggestion if you want something like that.