How to Merge Results Of an Union All In Oracle?

12 minutes read

To merge the results of an UNION ALL in Oracle, you can use a subquery or a temporary table to store the results of the UNION ALL operation, and then apply the merge statement to merge the results as needed. The MERGE statement enables you to specify a condition to determine whether to update existing rows in a target table or insert new rows into the table. By using MERGE, you can efficiently perform insert, update, or delete operations on a target table based on the results of a SELECT statement. Additionally, you can use the MERGE statement to combine data from multiple sources into a single table in Oracle.

Best Oracle Database Books To Read in July 2024

1
Oracle Database 12c DBA Handbook (Oracle Press)

Rating is 5 out of 5

Oracle Database 12c DBA Handbook (Oracle Press)

2
Oracle PL/SQL by Example (The Oracle Press Database and Data Science)

Rating is 4.9 out of 5

Oracle PL/SQL by Example (The Oracle Press Database and Data Science)

3
Oracle PL/SQL Programming: Covers Versions Through Oracle Database 12c

Rating is 4.8 out of 5

Oracle PL/SQL Programming: Covers Versions Through Oracle Database 12c

4
Expert Oracle Database Architecture: Techniques and Solutions for High Performance and Productivity

Rating is 4.7 out of 5

Expert Oracle Database Architecture: Techniques and Solutions for High Performance and Productivity

5
OCA Oracle Database SQL Exam Guide (Exam 1Z0-071) (Oracle Press)

Rating is 4.6 out of 5

OCA Oracle Database SQL Exam Guide (Exam 1Z0-071) (Oracle Press)

6
Oracle Database 12c SQL

Rating is 4.5 out of 5

Oracle Database 12c SQL

7
Modern Oracle Database Programming: Level Up Your Skill Set to Oracle's Latest and Most Powerful Features in SQL, PL/SQL, and JSON

Rating is 4.4 out of 5

Modern Oracle Database Programming: Level Up Your Skill Set to Oracle's Latest and Most Powerful Features in SQL, PL/SQL, and JSON

8
Oracle Database Administration: The Essential Refe: A Quick Reference for the Oracle DBA

Rating is 4.3 out of 5

Oracle Database Administration: The Essential Refe: A Quick Reference for the Oracle DBA

9
Practical Oracle SQL: Mastering the Full Power of Oracle Database

Rating is 4.2 out of 5

Practical Oracle SQL: Mastering the Full Power of Oracle Database


How to merge results of an union all in Oracle?

To merge the results of a UNION ALL in Oracle, you can use a SELECT statement with the UNION ALL operator followed by an ORDER BY clause to sort the merged results if needed. Here is an example:

1
2
3
4
5
6
SELECT * FROM (
    SELECT column1, column2 FROM table1
    UNION ALL
    SELECT column1, column2 FROM table2
) 
ORDER BY column1;


In this example, the results of the UNION ALL of table1 and table2 will be merged and sorted based on column1. You can adjust the SELECT statement and ORDER BY clause based on your specific requirements.


How to schedule and automate the merge operation using union all in Oracle?

To schedule and automate the merge operation using union all in Oracle, you can create a stored procedure and then use Oracle's scheduler to run this stored procedure at scheduled intervals. Here is a step-by-step guide to achieve this:

  1. Create a stored procedure that performs the merge operation using union all. Here is an example of a stored procedure that merges data from two tables using union all:
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
CREATE OR REPLACE PROCEDURE merge_tables AS
BEGIN
  MERGE INTO target_table tt
  USING (
    SELECT * FROM source_table1
    UNION ALL
    SELECT * FROM source_table2
  ) st ON (tt.id = st.id)
  WHEN MATCHED THEN
    UPDATE SET tt.column1 = st.column1, tt.column2 = st.column2
  WHEN NOT MATCHED THEN
    INSERT (id, column1, column2)
    VALUES (st.id, st.column1, st.column2);
END;
/


  1. Once you have created the stored procedure, you can schedule it to run at specified intervals using Oracle's DBMS_SCHEDULER package. Here is an example of scheduling the merge_tables procedure to run every day at 1:00 AM:
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
BEGIN
  DBMS_SCHEDULER.create_job (
    job_name        => 'MERGE_TABLES_JOB',
    job_type        => 'PLSQL_BLOCK',
    job_action      => 'BEGIN merge_tables; END;',
    start_date      => SYSTIMESTAMP,
    repeat_interval => 'FREQ=DAILY; BYHOUR=1',
    enabled         => TRUE
  );
END;
/


  1. You can also monitor the scheduled job using the DBMS_SCHEDULER views. For example, to view the details of the MERGE_TABLES_JOB, you can run the following query:
1
2
3
SELECT job_name, start_date, repeat_interval, enabled
FROM user_scheduler_jobs
WHERE job_name = 'MERGE_TABLES_JOB';


By following these steps, you can schedule and automate the merge operation using union all in Oracle. This will ensure that the merge operation is performed regularly and efficiently without manual intervention.


How to handle null values when merging results with union all in Oracle?

When using UNION ALL in Oracle to merge results from two or more queries, you will need to handle null values appropriately to ensure that the merged results are correct. Here are some ways to handle null values when merging results with UNION ALL in Oracle:

  1. Use COALESCE function: You can use the COALESCE function to replace null values with a specified default value. For example, if you want to replace null values in a column called "name" with the value "Unknown", you can use the following query:
1
2
3
SELECT COALESCE(name, 'Unknown') AS name FROM table1
UNION ALL
SELECT COALESCE(name, 'Unknown') AS name FROM table2;


  1. Use NULLIF function: You can use the NULLIF function to replace a specified value with null. For example, if you want to replace 'N/A' values in a column called "status" with null values, you can use the following query:
1
2
3
SELECT NULLIF(status, 'N/A') AS status FROM table1
UNION ALL
SELECT NULLIF(status, 'N/A') AS status FROM table2;


  1. Handle null values in the application layer: Another option is to handle null values in the application layer where the data is being processed. You can check for null values and perform any necessary transformations or replacements before merging the results using UNION ALL.


By using these approaches, you can handle null values appropriately when merging results with UNION ALL in Oracle.


What is the impact on resource consumption when merging results using union all in Oracle?

When merging results using UNION ALL in Oracle, the impact on resource consumption can vary depending on the specific query and data being merged. However, in general, using UNION ALL can have the following impacts on resource consumption:

  1. Increased memory usage: When using UNION ALL, Oracle needs to store the results of multiple queries in memory before merging them. This can result in increased memory usage, particularly if the queries being merged return a large amount of data.
  2. Increased CPU usage: Merging results using UNION ALL requires additional processing power to combine the results of multiple queries. This can result in increased CPU usage, particularly if the queries being merged are complex or return a large amount of data.
  3. Increased disk I/O: If the data being merged is not already in memory, Oracle may need to read data from disk to perform the merge. This can result in increased disk I/O and potentially slower query performance.


Overall, while using UNION ALL can be a useful tool for combining results from multiple queries, it is important to consider the potential impacts on resource consumption and query performance. It is recommended to carefully consider the size and complexity of the data being merged, as well as the available resources on the server, when using UNION ALL in Oracle.


How to use subqueries in the merged results of union all in Oracle?

To use subqueries in the merged results of a UNION ALL statement in Oracle, you need to wrap the subquery in parentheses and give it an alias. Here is an example:

1
2
3
4
5
6
7
8
9
SELECT column1, column2
FROM (
  SELECT column1, column2
  FROM table1
  UNION ALL
  SELECT column3, column4
  FROM table2
) subquery_alias
WHERE column1 = 'value';


In this example, the subquery is enclosed in parentheses and given an alias "subquery_alias". The outer query then selects the columns from this subquery and applies any additional filtering or sorting as needed.


Make sure to alias your subquery result and reference it correctly in your main query to use subqueries in the merged results of UNION ALL in Oracle.


What is the impact on query optimization when using union all in Oracle?

In Oracle, using UNION ALL can impact query optimization in several ways:

  1. Increased complexity: When using UNION ALL, Oracle may have to consider multiple subqueries or tables to combine the results, which can increase the complexity of the query optimization process.
  2. Performance impact: While UNION ALL can be faster than UNION because it does not remove duplicate rows, it can still impact query performance if not used efficiently. If the columns and datatypes of the tables being combined are not compatible, Oracle may need to perform additional operations to consolidate the results.
  3. Index usage: When using UNION ALL, Oracle may not be able to take advantage of indexes on the tables being queried, as the results may need to be sorted and combined before returning to the user. This can result in slower query performance compared to using individual queries.
  4. Query plan changes: The query execution plan generated by the Oracle optimizer may be different when using UNION ALL, as it needs to consider the differences in the tables being combined and the requirements of the query. This can result in suboptimal performance if the optimizer is not able to accurately estimate the cost of combining the results.


Overall, while UNION ALL can be a useful tool for combining results from multiple tables, it is important to consider the potential impact on query optimization and performance when using it in Oracle. Proper indexing, data compatibility, and efficient query structuring are key factors to consider when using UNION ALL to optimize query performance.

Facebook Twitter LinkedIn Whatsapp Pocket

Related Posts:

To merge pandas DataFrames on multiple columns, you can use the pd.merge() function and specify the columns to merge on by passing a list of column names to the on parameter. This will merge the DataFrames based on the values in the specified columns. You can ...
To merge or join two pandas DataFrames, you can use the merge() function. This function allows you to combine two DataFrames based on a common column or index. You can specify the type of join (inner, outer, left, or right) and the key column(s) to join on. Th...
To combine 2 select statements in Oracle, you can use the UNION keyword. This keyword allows you to combine the results of two separate SELECT statements into a single result set. Each SELECT statement must have the same number of columns in the same order. UN...