How to Efficiently Retrieve Matching Data Across Tables: Inner Join vs. Union?

Comparing Inner Join and Union for Cross-Table Data Matching

Retrieving matching data across multiple tables often requires careful consideration of database query methods. This article contrasts the use of INNER JOIN and UNION for this task, focusing on scenarios where they are most effective.

INNER JOIN vs. UNION

• INNER JOIN: This operation links rows from two or more tables based on matching values in specified columns. The result set only contains rows where a match exists in all joined tables.
• UNION: This combines the result sets of two or more SELECT statements into a single result set, eliminating duplicate rows. It's crucial that the number and data types of columns in the combined statements are identical.

Illustrative Example

Consider two tables:

• tbl_facilitators: Contains facilitator information.
• tbl_facilitatorClasses: Contains class details, including primary and secondary facilitator IDs.

The objective is to retrieve the class name and the first and last names of both primary and secondary facilitators for each class.

INNER JOIN Solution

The following query uses INNER JOIN to achieve this:

<code class="language-sql">SELECT
  tbl_facilitatorClasses.className,
  tbl_facilitators.facilLname AS primary_facilitator_lname,
  tbl_facilitators.facilFname AS primary_facilitator_fname,
  tbl_facilitators2.facilLname AS secondary_facilitator_lname,
  tbl_facilitators2.facilFname AS secondary_facilitator_fname
FROM tbl_facilitatorClasses
INNER JOIN tbl_facilitators
  ON tbl_facilitatorClasses.primeFacil = tbl_facilitators.facilID
INNER JOIN tbl_facilitators AS tbl_facilitators2
  ON tbl_facilitatorClasses.secondFacil = tbl_facilitators2.facilID;</code>

This query efficiently joins tbl_facilitatorClasses with tbl_facilitators twice (using an alias for the second join) to retrieve the required information for both primary and secondary facilitators in a single, structured result.

UNION Limitations

A UNION operation is unsuitable for this scenario. A UNION would require restructuring the data significantly, producing a less efficient and less readable result, and not providing the desired combined row structure for each class.

Conclusion

For retrieving related data from multiple tables based on matching column values, as demonstrated in this example, INNER JOIN is the far more appropriate and efficient choice compared to UNION. UNION is better suited for combining distinct result sets from similar queries, not for joining data based on relational keys.

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