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Most database management systems restrict check constraints to a single row, with access to constants and deterministic functions, but not to data in other tables, or to data invisible to the current transaction because of [[transaction isolation]].
Such constraints are not truly ''table check constraints'' but rather ''row check constraints''. Because these constraints are generally only verified when a row is directly updated (for performance reasons,) and often implemented as implied <code>INSERT</code> or <code>UPDATE</code> triggers, [[integrity constraints]] could be violated by indirect action were it not for these limitations.
* <code>CHECK ((select count(*) from invoices where invoices.customerId = customerId) < 1000)</code>
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