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How Can I Perform a Tableless UNION Query in Microsoft Access (Jet/ACE)?

Patricia Arquette
Patricia ArquetteOriginal
2025-01-12 07:39:43690browse

How Can I Perform a Tableless UNION Query in Microsoft Access (Jet/ACE)?

Performing UNION Queries Without Tables in Microsoft Access (Jet/ACE)

Microsoft Access, utilizing the Jet/ACE database engine, presents a unique obstacle when attempting tableless UNION queries. While straightforward SELECT statements without data sources are acceptable, UNION queries necessitate a FROM clause, regardless of whether it references any specific fields.

Underlying Reason:

The Jet/ACE engine differs from other database systems; it mandates at least one table or query as input for UNION operations. This constraint is implemented to maintain database stability and data integrity by establishing a link to the underlying data structure.

Circumventing the Limitation:

Several approaches can be employed to bypass this limitation:

  • Creating a Dummy Table:

    • Construct a table containing a single row and a primary key.
    • Populate the row with any suitable data.
    • Utilize this table in the FROM clause of your UNION query.
  • Employing Restricted SELECT Statements:

    • Utilize SELECT statements incorporating TOP 1 or specific WHERE clauses to limit the result set to a single row.
    • Substitute the UNION query with these modified SELECT statements.
  • Illustrative Example:

The following demonstrates a functional example leveraging the dummy table method:

<code class="language-sql">SELECT "Mike" AS FName
FROM Dual
UNION ALL
SELECT "John" AS FName
FROM Dual;</code>

Here, "Dual" represents the single-row table.

It's important to note that check constraints, designed to enforce a single-row limitation, were introduced with Jet 4 and are only compatible with ADO execution. DAO lacks the capability to create check constraints and necessitates explicit restrictions within the query itself.

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