I have a connection pool for database A and database B. I am moving some Node.JS code over to Go (I'm using SQL Server if that matters), and some of the queries are doing this:
db.A.Query(`
select ... from some_table;
select ... from B..other_table;
`)
Is it better to do it that way, or like:
db.A.Query(...)
db.B.Query(...)
I read this line:
create one sql.DB object for each distinct datastore you need to access
from here. And only now do I realize I read 'datastore' as 'database', so now I'm not even sure if it's efficient to have these two database connection pools!
Thank you for any help!
For most scenarios and SQL Server client programs sending multiple SELECT queries in a batch is not materially more efficient. Perhaps if the queries returned very small result sets, and you ran them at very high frequency, you could see a material difference. But in the paradigm case, whether you send the queries in one or two batches won't matter much.
It won't matter to SQL Server at all, so the only difference will be in the client/server network traffic.
SSMS will let you compare the client statistics between running queries in a one-batch script and a multi-batch script. EG running
select top 10 * from sys.objects
select top 5 * from sys.columns
and then
select top 10 * from sys.objects
GO
select top 5 * from sys.columns
in SSMS outputs the following client statistics
I have 2 databases with the same tables.
Both databases are on different SQL servers.
I added the second SQL server as a linked server, which works fine.
I want to run a simple DELETE on the linked DB (so that ID's that aren't on the local DB will be deleted).
When I have both DB's on the same server, it works
DELETE FROM TM.dbo.Departments
WHERE NOT EXISTS (SELECT * FROM SPO.dbo.Departments
WHERE TM.dbo.Departments.DepartmentID = spo.dbo.Departments.DepartmentID);
But when I try this on the Linked Server, it looks like this
DELETE FROM [LINKEDSRV].[TM].[dbo].[Departments]
WHERE NOT EXISTS (SELECT * FROM SPO.dbo.Departments WHERE
spo.dbo.Departments.DepartmentID = [LINKEDSRV].[TM].[dbo].[Departments].DepartmentID)
And the last line with is where I can't get it to work.
I hope you guys have a suggestion!
Try this:
DELETE LS
FROM [LINKEDSRV].[TM].[dbo].[Departments] LS
WHERE NOT EXISTS (SELECT 1 FROM SPO.dbo.Departments D
WHERE D.DepartmentID = LS.DepartmentID)
As I mentioned in the comments, 3+ naming for columns is deprecated (i.e. schema.object.column). Alias your objects and then prefix the column name with that for succinct and readable SQL.
I would like to execute SQL query from R and get results into dataframe. Here is my example.
library(RODBC)
db.handle <-odbcDriverConnect('driver=
{SQL Server Native Client 11.0};server=some_server\\some_server_2;
database = some_db;trusted_connection=yes')
query <-"select top 10 * into #temp_table from table_A select * from #temp_table"
res <- sqlQuery(db.handle, query)
print(res)
The above code return character(0). It works without #temp_table. Is there way to make it work with temp table?
While this may be a valid SQL statement, I'm not sure the the 'RODBC' package understands what to do with your query with the sqlQuery() command alone.
Assuming you have read and write permissions to the database you're using, try separating the functional operation into parts: sqlQuery(), sqlSave(), sqlQuery(). If you need to remove an existing temp table, you can use sqlDrop(). Don't forget to close the connection when finished.
Alternatively, you could run SQL like transformations in R with 'dplyr' after getting results from the 'select top 10 from table_A' query.
I'm running MS SQL Server and am trying to perform a JOIN between two tables located in different databases (on the same server). If I connect to the server using pyodbc (without specifying a database), then the following raw SQL works fine.
SELECT * FROM DatabaseA.dbo.tableA tblA
INNER JOIN DatabaseB.dbo.tableB tblB
ON tblA.id = tblB.id
Unfortunately, I just can't seem to get the analog to work using SQLAlchemy. I've seen this topic touched on in a few places:
Is there a way to perform a join across multiple sessions in sqlalchemy?
Cross database join in sqlalchemy
How do I connect to multiple databases on the same SQL Server with sqlalchemy?
How can I use multiple databases in the same request in Cherrypy and SQLAlchemy?
Most recommend to use different engines / sessions, but I crucially need to perform joins between the databases, so I don't think this approach will be helpful. Another typical suggestion is to use the schema parameter, but this does not seem to work for me. For example the following does not work.
engine = create_engine('mssql+pyodbc://...') #Does not specify database
metadataA = MetaData(bind=engine, schema='DatabaseA.dbo', reflect=True)
tableA = Table('tableA', metadataA, autoload=True)
metadataB = MetaData(bind=engine, schema='DatabaseB.dbo', reflect=True)
tableB = Table('tableB', metadataB, autoload=True)
I've also tried varients where schema='DatabaseA' and schema='dbo'. In all cases SQLAlchemy throws a NoSuchTableError for both tables A and B. Any ideas?
If you can create a synonym in one of the databases, you can keep your query local to that single database.
USE DatabaseB;
GO
CREATE SYNONYM dbo.DbA_TblA FOR DatabaseA.dbo.tableA;
GO
Your query then becomes:
SELECT * FROM dbo.DbA_TblA tblA
INNER JOIN dbo.tableB tblB
ON tblA.id = tblB.id
I'm able to run a test just like this here, reflecting from two remote databases, and it works fine.
Using a recent SQLAlchemy (0.8.3 recommended at least)?
turn on "echo='debug'" - what tables is it finding?
after the reflect all, what's present in metadataA.tables metadataB.tables?
is the casing here exactly what's on SQL server ? (e.g. tableA). Using a case sensitive name like that will cause it to be quoted.
While in Management Studio, I am trying to run a query/do a join between two linked servers.
Is this a correct syntax using linked db servers:
select foo.id
from databaseserver1.db1.table1 foo,
databaseserver2.db1.table1 bar
where foo.name=bar.name
Basically, do you just preface the db server name to the db.table ?
The format should probably be:
<server>.<database>.<schema>.<table>
For example:
DatabaseServer1.db1.dbo.table1
Update: I know this is an old question and the answer I have is correct; however, I think any one else stumbling upon this should know a few things.
Namely, when querying against a linked server in a join situation the ENTIRE table from the linked server will likely be downloaded to the server the query is executing from in order to do the join operation. In the OP's case, both table1 from DB1 and table1 from DB2 will be transferred in their entirety to the server executing the query, presumably named DB3.
If you have large tables, this may result in an operation that takes a long time to execute. After all it is now constrained by network traffic speeds which is orders of magnitude slower than memory or even disk transfer speeds.
If possible, perform a single query against the remote server, without joining to a local table, to pull the data you need into a temp table. Then query off of that.
If that's not possible then you need to look at the various things that would cause SQL server to have to load the entire table locally. For example using GETDATE() or even certain joins. Others performance killers include not giving appropriate rights.
See http://thomaslarock.com/2013/05/top-3-performance-killers-for-linked-server-queries/ for some more info.
SELECT * FROM OPENQUERY([SERVER_NAME], 'SELECT * FROM DATABASE_NAME..TABLENAME')
This may help you.
For those having trouble with these other answers , try OPENQUERY
Example:
SELECT * FROM OPENQUERY([LinkedServer], 'select * from [DBName].[schema].[tablename]')
If you still find issue with <server>.<database>.<schema>.<table>
Enclose server name in []
You need to specify the schema/owner (dbo by default) as part of the reference. Also, it would be preferable to use the newer (ANSI-92) join style.
select foo.id
from databaseserver1.db1.dbo.table1 foo
inner join databaseserver2.db1.dbo.table1 bar
on foo.name = bar.name
select * from [Server].[database].[schema].[tablename]
This is the correct way to call.
Be sure to verify that the servers are linked before executing the query!
To check for linked servers call:
EXEC sys.sp_linkedservers
right click on a table and click script table as select
select name from drsql01.test.dbo.employee
drslq01 is servernmae --linked serer
test is database name
dbo is schema -default schema
employee is table name
I hope it helps to understand, how to execute query for linked server
Usually direct queries should not be used in case of linked server because it heavily use temp database of SQL server. At first step data is retrieved into temp DB then filtering occur. There are many threads about this. It is better to use open OPENQUERY because it passes SQL to the source linked server and then it return filtered results e.g.
SELECT *
FROM OPENQUERY(Linked_Server_Name , 'select * from TableName where ID = 500')
For what it's worth, I found the following syntax to work the best:
SELECT * FROM [LINKED_SERVER]...[TABLE]
I couldn't get the recommendations of others to work, using the database name. Additionally, this data source has no schema.
In sql-server(local) there are two ways to query data from a linked server(remote).
Distributed query (four part notation):
Might not work with all remote servers. If your remote server is MySQL then distributed query will not work.
Filters and joins might not work efficiently. If you have a simple query with WHERE clause, sql-server(local) might first fetch entire table from the remote server and then apply the WHERE clause locally. In case of large tables this is very inefficient since a lot of data will be moved from remote to local. However this is not always the case. If the local server has access to remote server's table statistics then it might be as efficient as using openquery More details
On the positive side T-SQL syntax will work.
SELECT * FROM [SERVER_NAME].[DATABASE_NAME].[SCHEMA_NAME].[TABLE_NAME]
OPENQUERY
This is basically a pass-through. The query is fully processed on the remote server thus will make use of index or any optimization on the remote server. Effectively reducing the amount of data transferred from the remote to local sql-server.
Minor drawback of this approach is that T-SQL syntax will not work if the remote server is anything other than sql-server.
SELECT * FROM OPENQUERY([SERVER_NAME], 'SELECT * FROM DATABASE_NAME.SCHEMA_NAME.TABLENAME')
Overall OPENQUERY seems like a much better option to use in majority of the cases.
I have done to find out the data type in the table at link_server using openquery and the results were successful.
SELECT * FROM OPENQUERY (LINKSERVERNAME, '
SELECT DATA_TYPE, COLUMN_NAME
FROM [DATABASENAME].INFORMATION_SCHEMA.COLUMNS
WHERE
TABLE_NAME =''TABLENAME''
')
Its work for me
Following Query is work best.
Try this Query:
SELECT * FROM OPENQUERY([LINKED_SERVER_NAME], 'SELECT * FROM [DATABASE_NAME].[SCHEMA].[TABLE_NAME]')
It Very helps to link MySQL to MS SQL
PostgreSQL:
You must provide a database name in the Data Source DSN.
Run Management Studio as Administrator
You must omit the DBName from the query:
SELECT * FROM OPENQUERY([LinkedServer], 'select * from schema."tablename"')
For MariaDB (and so probably MySQL), attempting to specify the schema using the three-dot syntax did not work, resulting in the error "invalid use of schema or catalog". The following solution worked:
In SSMS, go to Server Objects > Linked Servers > Providers > MSDASQL
Ensure that "Dynamic parameter", "Level zero only", and "Allow inprocess" are all checked
You can then query any schema and table using the following syntax:
SELECT TOP 10 *
FROM LinkedServerName...[SchemaName.TableName]
Source: SELECT * FROM MySQL Linked Server using SQL Server without OpenQuery
Have you tried adding " around the first name?
like:
select foo.id
from "databaseserver1".db1.table1 foo,
"databaseserver2".db1.table1 bar
where foo.name=bar.name