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Microsoft has a bcp.exe program to copy a table from one SQL Server database to another, and there are SQL statements to SELECT INTO a new table or INSERT from a SELECT that will bulk copy data and even replicate table column definitions. However, the documentation makes it clear that if you want to move data from another type of database (like Oracle) you need some custom programming.
Java has its JDBC standard into which you plug drivers for various systems. .NET has essentially the same idea. However, while Java works on a row by row basis, .NET has some full table read and write operations that speed things up and transfer responsibility from the programmer to the drivers themselves. The simplest language in which to configure a table by table copy is Powershell.
While some Powershell programming was involved in writing the script, and may be needed to customize it if you require more than the basic operation, in most cases you just have to load the script and then write one copy command per table.
The input and output tables are assumed to be roughly identical. They would have the same column names and types that allow the data to be automatically converted. Specifically, the database systems and their client drivers will automatically convert any type of number to any other type of number big enough to hold the largest data value. Columns with types like if you link one database server to another you can copy both the definition and the contents of a table from one server to another using “SELECT INTO” or copy data from one table to another using “INSERT SELECT”.
However, things become more complicated if you are moving tables or data from one type of database to another. The requirement to migrate the Oracle ACS1 database to SQL Server produced a Powershell script that can be more generally used to synchronize information between databases and support Sandbox development and reporting.
Since our applications are written in Java, not use it? Powershell is based on the .NET framework, and while both Java and .NET have a similar plug in design for database drivers, .NET has full table operations that simply copying and migration.
The SQL for CREATE TABLE is much the same in all databases. The exact Types of columns may need adjustment because specific Type names mean different things in different systems. That is a manual process that you have to work out on your own. We assume you have generated an equivalent definition of each table in a target output system. The columns have the same name, and the types are similar enough that data can be automatically converted from one to the other. NUMERIC, DECIMAL, TINYINT, SMALLINT, INT, or BIGINT are compatible with each other, and different types of character strings (different sizes of numbers, CHAR, VARCHAR, VARCHAR2, NVARCHAR ) can be converted automatically although you may want to explicitly TRIM or declare character sets in the processare different character strings with different sizes and maybe character sets. Dates and timestamps will similarly convert even though different systems may use different starting dates and precisions.
In most cases the script can simply do a SELECT * FROM the input table and write the data to the output tableThere is no automatic way to handle data types like Blobs, Clobs, XML, etc. If you have special requirements, you can provide a custom SELECT statement. For example, in the special case where you are copying data from a production database to a test system (especially a developer’s Sandbox) you may want to avoid copying the real values of any sensitive information. You can then specify a SELECT that gives everyone in the output table the same Social Security Number or Date of Birth if the real columns of these types, you may need to write special conversion rules anyway.
While we want to copy the exact data between production systems, when we are moving data to a TEST or developer Sandbox, we may want to audit or transform the contents. For testing, we may want to assign a single common dummy SSN or Date of Birth to everyone, since the actual values are not needed for testing .The tool was built to migrate an Oracle database to SQL Server. Initially the Oracle data is copied to a development Sandbox. Later it is copied to pre-production. As the system rolls out, different tables may migrate at different times. Some authoritative Oracle tables will be copied to shadow SQL Servers table to support testing, while other already migrated SQL Server tables will be copied back to Oracle to support legacy applications or reports that have not yet been convertedand we then reduce the sensitivity of the copied tables.
During initial development the script can be used to periodically refresh the important tables from a source database to another. However, as we start to migrate application, different components will move at different times. The tool can synchronize some Oracle tables to SQL Server (to test new code) and some SQL Server tables back to Oracle (to support unconverted legacy applications and reports after the production application has moved to SQL Server).
The script can also be used to capture a whole database at points in time, then compare two snapshots to report what changed over time. The base version of the script has no special support for data types like Blobs, Clobs, XML, etc. If you have columns of these types, you may need to write special conversion rules anyway.for ad hoc reporting. For example, an analyst can capture point int time snapshots of data, compare the before and after, and generate reports on the changes over time.
Read and Write entire Tables
.NET defines some abstract classes of objects that perform generic functions. Oracle and SQL Server provide drivers that implement these template classes.
Two .NET templates that Java JDBC does not have are the DataAdapter and BulkCopy.
A DataAdapter will execute a SELECT and read You provide a text string containing a SELECT statement to a DataAdapter. It discovers the names and types of the columns from metadata supplied by the database, then reads the entire result into objects in memory. The . A NET program can then change specific values in the collection of Row objects in memory. The objects track what has been changed. A single operation tells the DataAdapter to generate work on the entire result as a Collection of objects.
The Row objects act like “Entities”. If you change data in a Row, or delete the Row, or add a new Row, the objects in memory track the changes you made. When you are done, you can tell the DataAdapter to alter the database table to match the changes you just made to the memory object, and the DataAdapter generates whatever INSERT, UPDATE, and DELETE operations are needed to transfer the changes previously made in memory to the database table.
If you give the DataAdapter a “SELECT *” statement, it will read metadata from the database and discover the names and types of each column. It then creates in memory native types for the client system that can hold any data that might be stored in that type of database column.
. (Of course, for this to work you either have to lock the result or be the only program making changes to it during the period).
Alternately, BulkCopy inserts new rows into a table, but it is optimized to transfer all the data rows in a single operation rather than doing a sequence of INSERT statements each of which has to be sent and then the return has to be checked. It can add new data, or if the table has been emptied it can load the table with entirely new BulkCopy can fill an empty table or append new rows after existing data. Triggers and Constraints are also disabled or optimized as you choose.
Since the objects in memory are the same no matter what database system you are using, .NET also defines templates for a DataSet, which is a generic collection of DataTables, which are generic collections of DataRows . However, while the DataAdapter and BulkCopy are implemented by the database drivers from the vendors (Oracle or SQL Server for example), the DataSet, DataTable, and DataRow are .NET classes independent of and generic to all databaseswhich are a collection of Items.
Normally, however, the script does not have to not deal with anything below the level of the entire table.
An example of the normal use of the script to copy all data in an Oracle table to a SQL Server table (which may have an older copy of the data in it) is to
Connect to the two databases with userids and password. The Oracle userid has to be able to read the table and the SQL Server userid has to be able to write to the table.
Have Oracle create a DataAdapter for the input table based on a “SELECT *”. This discovers the definition of the input table in the Oracle database including its column names and types.
Have SQL Server create a BulkCopy for the output table.
Create an empy empty DataSet object in memory.
Call the DataAdapter.fill() method passing the empty DataSet. This reads the table into memory generating one DataTable with DataRows containing the contents of the table. Oracle decides what native binary data types to use to hold each of the columnsvalues in each row.
Start a Transaction on the output database.
TRUNCATE the output table to delete all the old data.
Call BulkCopy.WriteToServer() passing the DataTable that Oracle generated. SQL Server converts whatever data Oracle put in memory to whatever type it needs to send the data to the database and repopulate its contentsAt this point the classes that implement the SQL Server BulkCopy object match the DataTable and DataRows created by Oracle to the column types in SQL Server and do any necessary type conversion, then send all the data out to the database to repopulate the table.
Commit the Transaction to swap the old and new version of the output table.
You can now close the connections or copy another table.
There are alternate Copy-Table DBTable functions or options of a function that combine steps 2 through 9. Any table can be emptied and repopulated.
If the tables have a unique key, then there is an option to read both the input and output tables into memory, compare the rows to see which ones are different, and only make the specific changes needed. However, it turns out that copying all the data is usually as fast or faster than comparing the tables. There is also a special function to append new data on the end of a table that doesn’t have a primary key. If you need some special transformation of the datathen use an output DataAdapter instead of a BulkCopy to only generate INSERT, UPDATE, or DELETE for the changed rows. This code had to be written to see if it was better, but it turned out that TRUNCATE and rewrite is just as fast and a lot simpler.
If the default version of the data provided by the input database doesn’t work, the best option is to create a string containing a specific SELECT statement using SQL functions to transform the data (SELECT … NULL AS SOCIAL_SECURITY_NUMBER …) and pass it to the Powershell command. If you need to do a more elaborate transformation, then you have to write some Powershell loop. Examples are provided in the code. However, in the normal case the Powershell script does not need any loops and does not look at individual rows let alone individual cells. The copy is done by complied code provided by Oracle and SQL Server. They choose the .NET data types and do any conversion needed. The script performs so little work itself that the copy runs as fast in Powershell as it would in any complied write a custom SELECT statement that transforms columns as they are read in. This avoids writing loops in Powershell. However, there are examples of using Powershell to examine values of selected columns of every row.
The normal case is just to let the database drivers do the work, and since they are the same drivers and this is the same .NET framework that every application would have to use, there is no performance loss using interpreted Powershell instead of a compiled language.
Use
Download the script from Git.
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. .\CopyTableRows.ps1
# Connect to an Oracle input database. For protection we separate the Userid (who should have read-only access) from the password
$inConnectionString = 'User Id={0};Password={1};Data Source=server.its.yale.edu/xxx' -f 'inuser', 'inpassword'
$inconn = New-Object Oracle.ManagedDataAccess.Client.OracleConnection($inConnectionString)
$inconn.Open() # Get a backing connection from the Oracle pool manager to reactivate the OracleConnection object
# Connect to a SqlSrv output database using the current Windows login as the credential
$outconn = New-Object System.Data.SqlClient.SqlConnection('Server=localhost;Integrated Security=true;Database=xxx;')
$outconn.Open()
Copy-DbTable -InputTableName 'EXAMPLE.TABLE' -InConn $inconn -OutConn $outconn -truncate
# Repeat as needed
$inconn.Close()
$outconn.Close()
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Considerations
Performance
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