How to Combine SQL Tables in Oracle Using UNION ALL

Discover how to effectively combine two SQL tables in Oracle using `UNION ALL`, while resolving common datatype issues.
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Combining SQL Tables in Oracle: A Step-by-Step Guide

When working with Oracle SQL, many developers find themselves needing to merge data from multiple tables. One common scenario is putting one SQL table "on top" of another. This article explores a problem many users face: how to combine two tables with identical column names without triggering datatype errors.

Understanding the Problem

Imagine you have two separate tables, table1 and table2, both containing the same column structure:

Table Structure

table1

column1: 1111111, aaaaaaa

column2: 2222222, bbbbbbb

column3: 3333333, ccccccc

table2

column1: 9999999, zzzzzzz

column2: 8888888, yyyyyyy

column3: 7777777, xxxxxxx

Your goal is to create a single output that vertically combines these two tables, producing a new dataset that retains the order of the entries.

The Challenge

You might be inclined to use a simple UNION query, such as:

[[See Video to Reveal this Text or Code Snippet]]

However, this could lead to errors like ORA-01790: expression must have same datatype as corresponding expression, which indicates that the datatypes of corresponding columns in the two tables don't match as expected.

The Solution: Using UNION ALL

To successfully combine both tables, we can use the UNION ALL command instead of just UNION. This approach not only combines the tables but also addresses the datatype issues. Here's a practical solution:

Revised SQL Query

[[See Video to Reveal this Text or Code Snippet]]

Breaking Down the Query

Selecting Columns: We explicitly select the column names. This is crucial because it ensures that the output maintains consistent datatypes across the tables.

Using UNION ALL: This command combines the results while keeping all records, including duplicates if applicable, and should bypass the error you're encountering.

Creating a Source Indicator: Introducing a computed column (src) allows us to maintain the original order of the data from table1 and table2. Values of 1 and 2 correspond to entries from each respective table.

Ordering the Results: Finally, we sort the entire result by the src column, which ensures the data from table1 appears first in the final output.

Expected Outcome

When executed correctly, this query will yield a combined output that appears like this:

column1column2column3111111122222223333333aaaaaaabbbbbbbccccccc999999988888887777777zzzzzzzyyyyyyyxxxxxxxConclusion

Combining multiple SQL tables can initially seem daunting, especially when faced with datatype errors. However, using the corrected UNION ALL approach not only resolves these issues but also presents a clean and orderly result. By following the provided steps, you can easily merge tables in Oracle and continue your database management tasks efficiently.

Final Thoughts

If you encounter any more challenges while dealing with SQL table combinations, remember that understanding the underlying data structure is key. Take your time to analyze the column types and ensure compatibility.