Inner Join vs. Union in Microsoft Access: When Should I Use Each to Retrieve Data from Multiple Tables?

Microsoft Access data retrieval: application scenarios of INNER JOIN and UNION

In a Microsoft Access database, you may need to retrieve data from multiple tables with related relationships. This can be achieved with INNER JOIN or UNION, which method you choose depends on your specific needs.

INNER JOIN (inner connection)

INNER JOIN is used to retrieve records with matching rows from multiple tables, based on specified criteria. For example, you need to retrieve data from the tbl_facilitators and tbl_facilitatorClasses tables, based on the primeFacil and secondFacil columns respectively.

INNER JOIN uses the ON keyword to specify matching conditions. Here is a sample query:

<code class="language-sql">SELECT tbl_facilitatorClasses.className,
    tbl_facilitators.facilLname, tbl_facilitators.facilFname
FROM tbl_facilitatorClasses
INNER JOIN tbl_facilitators
ON tbl_facilitatorClasses.primeFacil = tbl_facilitators.facilID;</code>

This query retrieves the course name, primary instructor's last name, and first name for records whose tbl_facilitatorClasses value in primeFacil matches the tbl_facilitators value in facilID.

UNION

UNION combines the results of two or more queries into a single result set. In this example, you use UNION to retrieve data for primary and secondary counselors separately. However, you need to manually combine the results to create the desired output.

Solution

In order to retrieve the primary and secondary counselor data in the format you expect, multiple INNER JOINs are required. The query in the solution you provided is correct:

<code class="language-sql">SELECT tblCLS.className,
    tblP.facilLname, tblP.facilFname, tblS.facilLname, tblS.facilFname
FROM (tbl_facilitatorClasses AS tblCLS
INNER JOIN tbl_facilitators AS tblP
ON tblCLS.primeFacil=tblP.facilID)
INNER JOIN tbl_facilitators AS tblS
ON tblCLS.secondFacil=tblS.facilID;</code>

This query uses parentheses to surround the first INNER JOIN to ensure correct order of operations and by matching the tbl_facilitatorClasses and primeFacil columns in secondFacil to the tbl_facilitators column in facilID to retrieve the required output for the primary and secondary counselors.

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