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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:
FROM
clause of your UNION query.Employing Restricted SELECT Statements:
SELECT
statements incorporating TOP 1
or specific WHERE
clauses to limit the result set to a single row.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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