Here is my code for joining two tables:
DB.from(:sources).join(:payloads, :source_id => :id)
The table names are :sources, :payloads.
The problem is that there is an :id column in payloads which overwrites the :id column in :sources. I need to use an alias so that I just obtain a mega table with all of the column names. However, as currently written and as my tables are currently structured, the :id columns are getting combined and the second table takes precedence. Does this make sense?
How do I make an alias so that the :id column from :sources still shows up?
To alias sources.id to a different name, use the Identifier aliases.
.select_append(:sources__id___source_id).join...
# *, `sources`.`id` AS 'source_id'
I think this is a case where using Sequel's graph will help you.
From the documentation:
Similar to #join_table, but uses unambiguous aliases for selected columns and keeps metadata about the aliases for use in other methods.
The problem you're seeing is an identically named column in one table is colliding with the same column name in another. Sequel's use of graph should make sure that the table name and column are returned as the key, rather than just the column.
The various documentation files have a number of examples, which would make a really long answer, so I recommend going through the docs, searching for uses, and see how they work for you.
Also, the Sequel IRC channel can be a great asset for these sort of questions too.
Related
I am having some trouble with a query in Crystal 2008. I have two tables with columns that are loosely related, both contain addresses. One table column is just a street name while the other is a street name plus some additional info. I want to find all records where these have the same street name and only show those. Example below:
Address
AddressB
123 St
123 St, ABC City
123 St
345 St, ABC City
I have tried using a formula such as below
if({AddressB} startswith {Address}) then {AddressB} else 'ERROR'
I have also tried this with LIKE and as well as * wildcards. Nothing seems to work. I will admit I am pretty amateur-ish with SQL and crystal so formulas are a new frontier for me writing reports. Also I should note that tables are linked appropriately with inner joins.
Any help would be greatly appreciated!
This should work. Perhaps your {Address} column is padded with spaces, so try:
IF ({AddressB} startswith Trim({Address})) THEN {AddressB} ELSE 'ERROR'
Test the effect of replacing the reference to the column name with the static text value that you "think" is in that column.
If you get a different behavior, what you think is in that column is not what is actually in that column. For example, the column might contain non-printable characters. You can get rid of those using the Replace function.
If you don't get a different behavior, then show us the expression with the static text values. That would allow us to replicate the behavior and understand the situation.
Note: the problem might be in your table join logic. If you have no join condition, then all records in TableA would join to all the records in TableB. In that case, you need to place the fields in the detail section to get a proper sense of what is being compared to what. Or rethink your join logic. Perhaps you should move one table to a subreport, or a SQL Expression instead of trying to include both tables in the main report.
I have patients table
It's contain smoking field it's values will be (smoker or non-smoker or ex-smoker)
My question is should I save values directly as string in patient table or make another table for smoking and make one-to-one relation between these tables?
Is it affects performance of query
When I have three options like this, I typically use a nullable bit/bool. A bit/bool column will be more performant than a string column.
So, in your situation, my table would have a "smoking" column where null would be non-smoker, 0/false would be ex-smoker, and 1/true would be smoker.
If you think you might add options later, like maybe "recently quit" or something, a nullable bool might not be ideal as you'd have to alter the column, then write a script to convert all of them into the appropriate string.
If you don't want to use the nullable bool, then you'd want a one to many table where you listed each option: "smoker, non-smoker, ex-smoker" then add a foreign key onto the "smoking" table in your patients table. That would also increase performance over throwing strings in the column.
I have installed hbase and I have access to command's shell.
I have a table with 2 familly column like this:
create 'arbres', 'emplacement', 'propriete'
This request works fine :
scan 'arbres',{FILTER=>"SingleColumnValueFilter('emplacement', 'lieu_adresse', =,'binary:VOIE INCONNUE')", COLUMNS=>['emplacement'], COLUMN=>15}
But this second one, list all rows, without filter
scan 'arbres',{FILTER=>"SingleColumnValueFilter('emplacement', 'lieu_adresse', =,'binary:VOIE INCONNUE')", COLUMNS=>['propriete'], COLUMN=>15}
I don't understand why and I don't find the reason in the documentation.
Please can you explain a little the reason.
regards
The second command has a filter on different column family and column that you are not accessing.
The push down requires the columns to be accessed, meaning you should have the column family and column mentioned in the COLUMNS=>[]
The reason one would have two different column families is to make access easier and light weight, since each column family will have its own file.
Background
I'm writing an adapter for ESE to .NET and LINQ in a Google Code project called eselinq. One important function I can't seem to figure out is how to get a list of indexes defined for a table. I need to be able to list available indexes so the LINQ part can automatically determine when indexes can be used. This will allow much more efficient plans for user queries if appropriate indexes can be found.
There are two related functions for querying index information:
JetGetTableIndexInfo - get index information by tableID
JetGetIndexInfo - get index information by tableName
These only differ in how the related table is specified (name or tableid). It sounds like these would support the function I want but all the info levels seem to require that I already have a certain index to query information for. The only exception is JET_IdxInfoCount, but that only counts how many indexes are present.
JET_IdxInfo with its JET_INDEXLIST sounds plausible but it only lists the columns on a specific index.
Alternatives
I am aware that I could get the index information another way, like annotations on .NET types corresponding to database tables, or by requiring a index mapping be provided ahead of time. I think there's enough introspection implemented to make everything else work out of the box without the user supplying extra information, except for this one function.
Another option may be to examine the system tables to find related index objects, but this is would mean depending on an undocumented interface.
To satisfy this question, I want a supported method of enumerating the indexes (just the name would be sufficient) on a table.
You are correct about JetGetTableIndexInfo and JetGetIndexInfo and JET_IdxInfo. The twist is that the data is returned in a somewhat complex: a temporary table is returned containing a row for the index and then a row for each column in the table. To just get the index names you will need to skip the column rows (the column count is given by the value of the columnidcColumn column in the first row).
For a .NET example of how to decipher this, look at the ManagedEsent project. In the MetaDataHelpers.cs file there is a method called GetIndexInfoFromIndexlist that extracts all the data from the temporary table.
How can I tell Activerecord to not load blob columns unless explicitly asked for? There are some pretty large blobs in my legacy DB that must be excluded for 'normal' Objects.
I just ran into this using rail 3.
Fortunately it wasn't that difficult to solve. I set a default_scope that removed the particular columns I didn't want from the result. For example, in the model I had there was an xml text field that could be quite long that wasn't used in most views.
default_scope select((column_names - ['data']).map { |column_name| "`#{table_name}`.`#{column_name}`"})
You'll see from the solution that I had to map the columns to fully qualified versions so I could continue to use the model through relationships without ambiguities in attributes. Later where you do want to have the field just tack on another .select(:data) to have it included.
fd's answer is mostly right, but ActiveRecord doesn't currently accept an array as a :select argument, so you'll need to join the desired columns into a comma-delimited string, like so:
desired_columns = (MyModel.column_names - ['column_to_exclude']).join(', ')
MyModel.find(id, :select => desired_columns)
I believe you can ask AR to load specific columns in your invocation to find:
MyModel.find(id, :select => 'every, attribute, except, the, blobs')
However, this would need to be updated as you add columns, so it's not ideal. I don't think there is any way to specifically exclude one column in rails (nor in a single SQL select).
I guess you could write it like this:
MyModel.find(id, :select => (MyModel.column_names - ['column_to_exclude']).join(', '))
Test these out before you take my word for it though. :)
A clean approach requiring NO CHANGES to the way you code else where in your app, i.e. no messing with :select options
For whatever reason you need or choose to store blobs in databases.
Yet, you do not wish to mix blob columns in the same table as your
regular attributes. BinaryColumnTable helps you store ALL blobs in
a separate table, managed transparently by an ActiveRecord model.
Optionally, it helps you record the content-type of the blob.
http://github.com/choonkeat/binary_column_table
Usage is simple
Member.create(:name => "Michael", :photo => IO.read("avatar.png"))
#=> creates a record in "members" table, saving "Michael" into the "name" column
#=> creates a record in "binary_columns" table, saving "avatar.png" binary into "content" column
m = Member.last #=> only columns in "members" table is fetched (no blobs)
m.name #=> "Michael"
m.photo #=> binary content of the "avatar.png" file