I need to join two tables Companies and Customers.
Companies table is in MS SQLServer and Customer table is in MySQL Server .
What is the best way to achieve this goal ?
If I am understand correctly, you need to join tables in SQL Server, not in code, because tag is sql.
If I have right, then you need to do some administrative tasks, like server linking.
Here you have an explanation how to link MySQL server into MSSQL server.
After you successfully link those servers, then your syntax is simple as:
SELECT
[column_list]
FROM companies
JOIN [server_name].[database_name].[schema_name].[table_name]
WHERE ...
Keep in mind that when accessing tables that exist on linked server, then you must write four-part names.
In order to query 2 databases, you need 2 separate connections. In this case, you would also need separate drivers, since you have a MSSQL and a MySQL database. Because you need separate connections, you need 2 separate queries. Depending on what you want to do, you can first retrieve your Companies and then do a query on Customers WHERE company = 'some value from COMPANIES' (or the other way around).
You could also just fetch every row from both tables in their own lists and compare those lists in your code, rather than using a query.
Try the following:
1 retrieve the data from Companies table from the SQL server and store the required columns in an ArrayList<HashMap<String,String>> format.
Therefore creating rows as arraylist index and HashMap as the key value pair responding to column names. Key: column name and Value as column value of that row.
2 Then pull data from Customer tables adding a where clause by converting data from your first Map into a comma separated format. Thus creating a filter similar to the join in SQL.
Add the data to the same result set data as before thus not over lapping the column names in the HashMap.
when u need to access the 5th row column7 then u write
ArrayList.get(4).get("column7");
The logic is given, Please implement it yourself.
Select Companies from DB1
Select Customers from DB2
Put them in Map<WhatToJoinOn, Company> and Map<WhatToJoinOn, Customer>
Join on map keys, creating a List<CompanyCustomer>
Related
I'm using SQL Server 2012. I want to join two tables without columns that I can join them, how can I find all the tables to reach to this two tables?
For example: I need to join the Table A to table D and to do that I need to connect A to B and then to C and at the end to D.
My question is: can I find the tables B and C among thousands of tables in the database without searching table by table?
Thanks a lot,
Ohad
Assuming that:
You want to automate this process
You have FOREIGN KEY constraints that you can rely on
You should proceed as follows:
Query sys.foreign_keys and create a directed graph structure that will contain the links between tables.
Then implement a graph search algorithm that will start from table A and try to find a path to table D and from D to A.
Once you have found the path, it will be easy to construct dynamic SQL containing the join of all tables on the path. You will need to query sys.foreign_key_columns as well to be able to construct the ON clauses of the JOIN's.
Let me know if you need help with more detail.
There's a couple of things you can do to help your cause, but for the most part, there's no direct way and you would need to know how your database is structured and the purposes of the tables. Furthermore, based on the database's design, it might be very difficult for you to intuitively find your answer and you might need just need to get guidance from someone who is knowledgeable with the database design. Regardless:
Fields in your tables A & D:
you can look at primary fields or unique fields in the tables to determine what other tables may link to those table. Usually they are named in a way that match those other tables and you can tell what table they're coming from.
Information_Schema Views
You can use information_schema.tables and information_schema.column views to easily search for names of tables and columns across the entire database and narrow your search to less tables.
I have two Microsoft Access 2007 databases. They both have tables that are very similar, there are a couple of columns that are different, but most of the structure is the same.
I exported one of the tables (table A) to Excel and filtered, getting a list of rows and their keys.
What I want to do is to update rows on the other table (table B) where the keys are the same, I can't filter using the same logic because table B does not have the columns needed to filter in the same way.
What will happen is some cell in table B will be set to the value in table A, if the key for the row matches a key on the filtered table A.
My idea was to import both tables into a C# application and make the changes programmatically, but if there is an easier way, possibly a SQL based way (Or using something in Excel) to update one of the tables based on a table in a different database, I would like to use it to be able to finish faster.
You can write cross database SQL query's in ms-access. This is based on file location. Explained here
So you could do the same with an update query.
UPDATE
s2006
SET
s2006.col1 = s2007.col1,
s2006.col2 = s2007.col2
FROM
c:\data\Sales2006.Sales s2006
INNER JOIN
c:\data\Sales2007.Sales s2007
ON
s2006.id = s2007.id
This sound to me like a situation to avoid however.
Is there a way to reference a table by id. Essentially, the table contains rows which pertain to one of many tables.
e.g. I need to reference tables A or B or C in table D's rows so that I know which table to join on.
I assume that if there is no shorthand way to reference a table externally then the only option is to store the table's name.
There is a "shorthand reference" for a table: the object identifier - the OID of the catalog table pg_class. Simple way to get it:
SELECT 'schema_name.tbl_name'::regclass
However, do not persist this in your user tables. OIDs of system tables are not stable across dump / restore cycles.
Also, plain SQL statements do not allow to parameterize table names. So you'll have to jump through hoops to use your idea in queries. Either dynamic SQL or multiple LEFT JOINs with CASE statements ... potentially expensive.
I am working on an asp.net application with SQL server database. This db has two tables Vacancies and dutystations. Vacancies table has a column named dutystationId which stores ids of dutystations in comma separated list like this:
2,12,15,18,19,23
Now I want to show this vacancy in grid and I have used left join like this:
QUERY
SELECT * FROM dbo.hr_Vacancies
CROSS APPLY dbo.hr_Split(dbo.hr_Vacancies.DutyStationID, ',') AS s
LEFT OUTER JOIN dbo.hr_DutyStations
ON s.Data = dbo.hr_DutyStations.DutyStationID
and in xsd, I have set vacancyid as primary key. but I get error:
ERROR
Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.
If I remove this constraint, I get 6 rows. I want to show one row only. How can I do this?
I stopped reading here:
Vacancies table has a column named dutystationId which stores ids of dutystations in comma seperated list
That is your problem right there. If you have comma separated values in an RDBMS, specifically if they contain foreign keys to other tables, you should halt full stop whatever you're doing and start redesigning your database. Many-to-many relations in an RDBMS are implemented with junction tables, and if you use them all your problems will suddenly solve themselves.
Your current design is not only hell to write SQL queries for, like this question illustraties perfectly as you cannot solve a trivial task, but it also kills performance - those calls to hr_Split are infinitely more computationally expensive than just doing proper joins.
Don't fall into the XY trap, solve the real problem first. Which is that you're even violating First Normal Form right now.
I have multiple database files which exist in multiple locations with exactly similar structure. I understand the attach function can be used to connect multiple files to one database connection, however, this treats them as seperate databases. I want to do something like:
SELECT uid, name FROM ALL_DATABASES.Users;
Also,
SELECT uid, name FROM DB1.Users UNION SELECT uid, name FROM DB2.Users ;
is NOT a valid answer because I have an arbitrary number of database files that I need to merge. Lastly, the database files, must stay seperate. anyone know how to accomplish this?
EDIT: an answer gave me the idea: would it be possible to create a view which is a combination of all the different tables? Is it possible to query for all database files and which databases they 'mount' and then use that inside the view query to create the 'master table'?
Because SQLite imposes a limit on the number of databases that can be attached at one time, there is no way to do what you want in a single query.
If the number can be guaranteed to be within SQLite's limit (which violates the definition of "arbitrary"), there's nothing that prevents you from generating a query with the right set of UNIONs at the time you need to execute it.
To support truly arbitrary numbers of tables, your only real option is to create a table in an unrelated database and repeatedly INSERT rows from each candidate:
ATTACH DATABASE '/path/to/candidate/database' AS candidate;
INSERT INTO some_table (uid, name) SELECT uid, name FROM candidate.User;
DETACH DATABASE candidate;
Some cleverness in the schema would take care of this.
You will generally have 2 types of tables: reference tables, and dynamic tables.
Reference tables have the same content across all databases, for example country codes, department codes, etc.
Dynamic data is data that will be unique to each DB, for example time series, sales statistics,etc.
The reference data should be maintained in a master DB, and replicated to the dynamic databases after changes.
The dynamic tables should all have a column for DB_ID, which would be part of a compound primary key, for example your time series might use db_id,measurement_id,time_stamp. You could also use a hash on DB_ID to generate primary keys, use same pk generator for all tables in DB. When merging these from different DBS , the data will be unique.
So you will have 3 types of databases:
Reference master -> replicated to all others
individual dynamic -> replicated to full dynamic
full dynamic -> replicated from reference master and all individual dynamic.
Then, it is up to you how you will do this replication, pseudo-realtime or brute force, truncate and rebuild the full dynamic every day or as needed.