In this article
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This article emphasizes the different scenarios that are possible while performing an SCD2 operation and the behavior of the ELT SCD2 Snap, in each of these scenarios.
Terminology
To follow these scenarios and configure the ELT SCD2 Snap properly in your SnapLogic environment, you will need to understand the following terms:
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Term
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Definition
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Target Table Natural Key
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A unique identifier based on which the SCD2 operation inserts a new record with different values for the same key in the target table.
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Target Table Cause-Historization Field(s)
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One or more fields in the target table which can cause new versions of data in the row.
For example, changes in Address or Employee Designation of an individual over a period of time. Each change in the address can cause a new version of the row (SCD Type2 entry) and so does each change in an employee’s designation.
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Target Table Temporal Fields
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A set of fields in the target table that store information specific to the validity of the cause-historization values in each of the row versions created. They can be a combination of the following fields:
Current row (Required): Boolean value to indicate if an entry is the current row.
Historical row (Required): Boolean value to indicate if an entry is a historical row.
Start date of current row (Required): Starting date-time for an entry to be the current row.
End date of current row (Optional) or End date of historical row (not chosen, by default): Date-time till when a current row entry remains current (becomes a historical row after that date-time passes by).
Invalid historical rows flag (Optional): An indicator for invalid rows in the target table. They are marked invalid due to lack of adequate historization information (logically incorrect values or null values) in the rows.
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For every natural key in the target table, there must be only one current row (valid now). All other entries (rows) for that natural key are either historical rows or invalid rows. A change of value for a cause-historization field causes the existing current row to expire and the new entry (with the updated value) becomes the current row. |
Scenario 1: Ideal SCD2 operation
Target table In this article
| Table of Contents | ||||||
|---|---|---|---|---|---|---|
|
This article emphasizes the different scenarios that are possible while performing an SCD2 operation and the behavior of the ELT SCD2 Snap, in each of these scenarios.
Terminology
To follow these scenarios and configure the ELT SCD2 Snap properly in your SnapLogic environment, you will need to understand the following terms:
Term | Definition | ||
|---|---|---|---|
Target Table Natural Key | A unique identifier based on which the SCD2 operation inserts a new record with different values for the same key in the target table. | ||
Target Table Cause-Historization Field(s) | One or more fields in the target table which can cause new versions of data in the row. For example, changes in Address or Employee Designation of an individual over a period of time. Each change in the address can cause a new version of the row (SCD Type2 entry) and so does each change in an employee’s designation. | ||
Target Table Temporal Fields | A set of fields in the target table that store information specific to the validity of the cause-historization values in each of the row versions created. They can be a combination of the following fields:
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Scenario 1: Ideal SCD2 operation
Target Table (Before)
Customer ID | Address | Start Date | End Date | Current Row |
1 | Chicago | 1/1/18 | 12/1/18 | false |
1 | New York | 1/1/19 | MAX_DATE | true |
Source/Update Table (Incoming SCD2 Data)
Customer ID (Natural Key) | Address (Cause Historization Values) | Start Date (Temporal Field 1) |
1 | Chicago | 1/1/20 |
1 | New York | 1/1/21 |
Output 1: Target Table (After SCD2 operation)
Customer ID (Natural Key) | Address (New Cause Historization Entries) | Start Date | End Date | Current Row | Remarks / Key Take-aways |
1 | Chicago | 1/1/18 | 12/1/18 | false | No change. |
1 | New York | 1/1/19 | 1/1/20 | false | Not current row anymore. |
1 | Chicago | 1/1/20 | 1/1/21 | false | New entry but not current row. Start date for the newer row is the End date for this entry. |
1 | New York | 1/1/21 | MAX_DATE | true | Current row with default End Date. |
| Info |
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The SCD2 operation uses a row-by-row approach. The final current row is arrived at, only after processing all rows for that natural key. |
Scenario 2: Cause Historization Row is Same as Current Row
Target Table (Before)
Customer ID | Address | Start Date | End Date | Current Row |
1 | Chicago | 1/1/18 | 12/1/18 | false |
1 | New York | 1/1/19 | MAX_DATE | true |
Source/Update Table (Incoming SCD2 Data)
Customer ID (Natural Key) | Address (Cause Historization Values) | Start Date (Temporal Field 1) |
1 | New York | 1/1/20 (or 1/1/17) |
1 | Boston | 1/1/21 |
Output 2: Target Table (After SCD2 operation)
Customer ID (Natural Key) | Address (New Cause Historization Entries) | Start Date | End Date | Current Row | Remarks / Key Take-aways |
1 | Chicago | 1/1/18 | 12/1/18 | false | No change. |
1 | New York | 1/1/19 | 1/1/21 | false | Not current row anymore. Any end date for New York before 1/1/21 (start date for Boston) results in New York becoming a historical row. |
1 | Boston | 1/1/21 | MAX_DATE | true | Current row with default End Date. |
Scenario 3: Target Table is Empty and Source Table Contains Duplicate Values for Cause-Historization field
Target Table (Before)
Customer ID | Address | Start Date | End Date | Current Row |
(Empty Table) |
Source/Update Table (Incoming SCD2 Data)
Customer ID (Natural Key) | Address (Cause Historization Values) | Start Date (Temporal Field 1) |
1 | Chicago | 1/1/15 |
1 | Boston | 1/1/16 |
1 | Chicago | 1/1/18 |
1 | Chicago | 1/1/19 (or 1/1/14) |
1 | Boston | 1/1/20 |
Output 3: Target Table (After SCD2 operation)
Customer ID (Natural Key) | Address (New Cause Historization Entries) | Start Date | End Date | Current Row | Remarks / Key Take-aways |
1 | Chicago | 1/1/15 | 1/1/16 | false | End date arrived based on the start date of the next entry (in ascending order). |
1 | Boston | 1/1/16 | 1/1/18 | false | End date arrived based on the start date of the next entry (in ascending order). |
1 | Chicago | 1/1/18 | 1/1/20 | false | Any start date for Chicago outside the range 1/1/18 - 1/1/20 will be ignored. |
1 | Boston | 1/1/20 | MAX_DATE | true | Current row with default End Date. |
| Info |
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The SCD2 operation sorts the source data records in the ascending order of their Start Dates when there is no row numbering (sequence) specified in the source data. |
Scenario 4: Start Date in Source Row Earlier than Start Date of Current Row
Target Table (Before)
Customer ID | Address | Start Date | End Date | Current Row |
1 | Chicago | 1/1/18 | 12/1/18 | false |
1 | New York | 1/1/19 | MAX_DATE | true |
Source/Update table Table (Incoming SCD2 Data)
Customer ID (Natural Key) | Address (Cause Historization Values) | Start Date (Temporal Field 1) |
1 |
Orlando | 1/1/ |
17 |
1 |
Boston | 1/1/21 |
Output 4.1: Target
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Table (After SCD2 operation) when Invalid rows are Ignored
Customer ID (Natural Key) | Address (New Cause Historization Entries) | Start Date | End Date | Current Row | Remarks / Key Take-aways |
1 | Chicago | 1/1/18 | 12/1/18 | false | No change. |
1 | New York |
1/1/19
1/1/20
false
Not current row anymore.
1
1/1/ |
19 | 1/1/21 | false |
Not current row anymore. |
Any end date for New York before 1/1/21 (start date for Boston) results in New York becoming a historical row. |
1 |
Boston | 1/1/21 | MAX_DATE | true |
Current row with default End Date.
| Info |
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The SCD2 operation uses a row-by-row approach. The final current row is arrived at, only after processing all rows for that natural key. |
Scenario 2: Cause Historization Row is Same as Current Row
Target table (Before)
Current row with default End Date. |
Output 4.2: Target Table (After SCD2 operation) when Invalid rows are Logged
Customer ID (Natural Key) | Address |
(New Cause Historization |
Entries) | Start Date | End Date | Current Row | Invalid | Remarks / Key Take-aways | |
1 | Chicago | 1/1/18 | 12/1/18 | false | No change. | |
1 | New York | 1/1/19 |
MAX_DATE
true
Source/Update table (Incoming SCD2 Data)
Customer ID (Natural Key)
Address (Cause Historization Values)
Start Date (Temporal Field 1)
1
1/1/21 | false | Not current row anymore. Any end date for New York before 1/1/21 (start date for Boston) results in New York becoming a historical row. | |
1 | Orlando | 1/1/ |
17 | MIN_DATE | false | true | Invalid row logged with End Date defaulted to MIN_DATE. |
1 | Boston | 1/1/21 |
...
MAX_DATE | true | Current row with default End Date. |
Scenario 5: One or more Start Dates in Source Rows contain NULLs
Target Table (Before)
Customer ID | Address |
Cause Historization |
Field) | Start Date | End Date | Current Row |
Remarks / Key Take-aways
1 | Chicago | 1/1/18 | 12/1/18 |
false
No change.
1
New York
1/1/19
1/1/21
false
false | |
1 |
New York | 1/1/ |
19 | MAX_DATE | true |
Current row with default End Date.
Scenario 3: Target table is Empty and Source Table Contains Duplicate Values for Cause-Historization field
Target table (Before)
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Source/Update Table (Incoming SCD2 Data)
Customer ID (Natural Key) | Address (Cause Historization Values) | Start Date (Temporal Field 1) |
1 | Orlando | NULL |
1 | Boston | 1/1/21 |
Output 5.1: Target Table (After SCD2 operation) when Invalid rows in the Source are Ignored
Customer ID (Natural Key) | Address |
(New Cause Historization |
Entries) | Start Date | End Date | Current |
(Temporal Field 3)
(Empty Table)
Source/Update table (Incoming SCD2 Data)
Customer ID (Natural Key)
Address (Cause Historization Values)
Row |
3) | Remarks / Key Take-aways | |
1 | Chicago | 1/1/ |
18 | 12/1/18 |
Boston
false | No change. |
1 |
New York | 1/1/ |
19 | 1/1/21 |
Chicago
false | Not current row anymore. Any end date for New York before 1/1/ |
21 (start date for Boston) results in New York becoming a historical row. | ||
1 | Boston | 1/1/ |
...
21 | MAX_DATE | true | Current row with default End Date. |
Output 5.2: Target Table (After SCD2 operation) when Invalid rows are Logged and Nulls Honored
Customer ID (Natural Key) | Address (New Cause Historization Entries) | Start Date | End Date | Current Row | Invalid | Remarks / Key Take-aways |
1 | Chicago | 1/1/ |
18 |
12/1/ |
18 | false |
No change. | |
1 |
New York | 1/1/ |
19 | 1/1/ |
21 | false |
Not current row anymore. Any end date for New York before 1/1/21 (start date for Boston) results in New York becoming a historical row. | |
1 |
Chicago
1/1/18
1/1/20
false
Orlando | NULL | MIN_DATE | false | true | NULL remains as is. Invalid row logged with End Date defaulted to MIN_DATE. |
1 | Boston | 1/1/ |
21 | MAX_DATE |
true
Current row with default End Date.
| Info |
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The SCD2 operation sorts the source data records in the ascending order of their Start Dates when there is no row numbering (sequence) specified in the source data. |
Scenario 4: Start Date in Source Row Earlier than Start Date of Current Row
...
true | Current row with default End Date. |
Output 5.3: Target Table (After SCD2 operation) when the Pipeline is set to Return the Error upon Detecting Invalid rows
No updates to the target table. SCD2 operation is aborted.
Scenario 6: NULLs in Natural Key or Cause Historization Fields at Source
Target Table (Before)
Customer ID | Address | Start Date | End Date | Current Row |
1 | Chicago | 1/1/18 | 12/1/18 | false |
1 | New York | 1/1/19 | MAX_DATE | true |
Source/Update table Table (Incoming SCD2 Data)
Customer ID (Natural Key) | Address (Cause Historization Values) | Start Date (Temporal Field 1) |
1 |
NULL | 1/1/ |
20 | ||
1 | Boston | 1/1/21 |
...
NULL | Miami | 1/1/22 |
Output 6.1: Target
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Table (After SCD2 operation) when
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NULLs in Source and Target Tables are Honored
Customer ID (Natural Key) | Address (New Cause Historization Entries) | Start Date | End Date | Current Row | Remarks / Key Take-aways |
1 |
Chicago | 1/1/18 | 12/1/18 | false | No change. |
1 | New York | 1/1/ |
19 |
1/1/ |
20 | false |
Not current row anymore. Any end date for New York before 1/1/20 (start date for the next row) results in New York becoming a historical row. |
1 |
NULL | 1/1/ |
20 | 1/1/21 | false |
A NULL entry made in the ascending order of the start dates. End Date added This row cannot be the current row |
. | ||
1 | Boston | 1/1/21 |
1
MAX_DATE | true | Current row with default End Date. | |
NULL | Miami | 1/1/ |
22 | MAX_DATE | true |
This entry, though included in the target table, does not correspond to the same Natural Key as the other entries, in this example. |
Output
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6.2: Target
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Table (After SCD2 operation) when
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NULLs in Source and Target Tables are Ignored
Customer ID (Natural Key) | Address (New Cause Historization Entries) | Start Date | End Date | Current Row | Invalid | Remarks / Key Take-aways |
1 | Chicago | 1/1/18 | 12/1/18 | false | No change. | |
1 | New York | 1/1/19 | 1/1/21 | false | Not current row anymore. Any end date for New York before 1/1/21 (start date for Boston) results in New York becoming a historical row. |
1
Orlando
1/1/17
MIN_DATE
false
true
Any entry (Start Date 1/1/20, in this case) containing a NULL is ignored. | ||||||
1 | Boston | 1/1/21 | MAX_DATE | true | Current row with default End Date. |