diff --git a/src/test/kotlin/kestrel/UniqueConstraintTest.kt b/src/test/kotlin/kestrel/UniqueConstraintTest.kt
new file mode 100644
index 0000000..3547261
--- /dev/null
+++ b/src/test/kotlin/kestrel/UniqueConstraintTest.kt
@@ -0,0 +1,157 @@
+package kestrel
+
+import kestrel.catalog.Catalog
+import kestrel.engine.Executor
+import kestrel.engine.InsertResult
+import kestrel.engine.SelectResult
+import kestrel.engine.StatementResult
+import kestrel.engine.UpdateResult
+import kestrel.sql.lexer.Lexer
+import kestrel.sql.parser.Parser
+import kestrel.storage.Value
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertTrue
+
+class UniqueConstraintTest {
+    private val catalog = Catalog()
+    private val executor = Executor(catalog)
+
+    private fun run(sql: String): List<StatementResult> =
+        Parser(Lexer(sql).tokenize()).parseProgram().map { executor.execute(it) }
+
+    private fun exec(sql: String): StatementResult = run(sql).single()
+
+    private fun select(sql: String): SelectResult = exec(sql) as SelectResult
+
+    private fun int(value: Long) = Value.IntegerValue(value)
+    private fun text(value: String) = Value.TextValue(value)
+
+    @Test
+    fun columnLevelUniqueRejectsDuplicateValue() {
+        run("CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT UNIQUE)")
+        run("INSERT INTO users VALUES (1, 'ada@example.com')")
+        val error = assertFailsWith<KestrelException> {
+            run("INSERT INTO users VALUES (2, 'ada@example.com')")
+        }
+        assertTrue(error.message!!.contains("UNIQUE constraint"), error.message!!)
+    }
+
+    @Test
+    fun columnLevelUniqueAllowsMultipleNullValues() {
+        run("CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT UNIQUE)")
+        run("INSERT INTO users VALUES (1, NULL)")
+        val result = exec("INSERT INTO users VALUES (2, NULL)")
+        assertEquals(InsertResult(1), result)
+    }
+
+    @Test
+    fun tableLevelCompositeUniqueRejectsDuplicateCombination() {
+        run(
+            "CREATE TABLE enrollments (" +
+                "student_id INTEGER, course_id INTEGER, term TEXT, " +
+                "UNIQUE (student_id, course_id, term))"
+        )
+        run("INSERT INTO enrollments VALUES (1, 10, 'fall')")
+        assertFailsWith<KestrelException> {
+            run("INSERT INTO enrollments VALUES (1, 10, 'fall')")
+        }
+    }
+
+    @Test
+    fun tableLevelCompositeUniqueAllowsDifferingCombinations() {
+        run(
+            "CREATE TABLE enrollments (" +
+                "student_id INTEGER, course_id INTEGER, term TEXT, " +
+                "UNIQUE (student_id, course_id, term))"
+        )
+        run("INSERT INTO enrollments VALUES (1, 10, 'fall')")
+        val bySameStudentDifferentCourse = exec("INSERT INTO enrollments VALUES (1, 11, 'fall')")
+        assertEquals(InsertResult(1), bySameStudentDifferentCourse)
+        val bySameStudentAndCourseDifferentTerm = exec("INSERT INTO enrollments VALUES (1, 10, 'spring')")
+        assertEquals(InsertResult(1), bySameStudentAndCourseDifferentTerm)
+    }
+
+    @Test
+    fun tableLevelCompositeUniqueExemptsRowsWithAnyNullColumn() {
+        run(
+            "CREATE TABLE enrollments (" +
+                "student_id INTEGER, course_id INTEGER, term TEXT, " +
+                "UNIQUE (student_id, course_id, term))"
+        )
+        run("INSERT INTO enrollments VALUES (1, 10, NULL)")
+        val result = exec("INSERT INTO enrollments VALUES (1, 10, NULL)")
+        assertEquals(InsertResult(1), result)
+    }
+
+    @Test
+    fun updateRejectsUniqueViolationAgainstAnotherRow() {
+        run("CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT UNIQUE)")
+        run("INSERT INTO users VALUES (1, 'ada@example.com'), (2, 'grace@example.com')")
+        assertFailsWith<KestrelException> {
+            run("UPDATE users SET email = 'ada@example.com' WHERE id = 2")
+        }
+    }
+
+    @Test
+    fun updateToSameRowsOwnValueIsNotAViolation() {
+        run("CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT UNIQUE)")
+        run("INSERT INTO users VALUES (1, 'ada@example.com')")
+        val result = exec("UPDATE users SET email = 'ada@example.com' WHERE id = 1")
+        assertEquals(UpdateResult(1), result)
+    }
+
+    @Test
+    fun uniqueConstraintRejectsUnknownColumn() {
+        assertFailsWith<KestrelException> {
+            run("CREATE TABLE t (id INTEGER PRIMARY KEY, UNIQUE (missing))")
+        }
+    }
+
+    @Test
+    fun foreignKeyCanReferenceAUniqueNonPrimaryKeyColumn() {
+        run("CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT UNIQUE)")
+        run("CREATE TABLE profiles (id INTEGER PRIMARY KEY, user_email TEXT REFERENCES users(email))")
+        run("INSERT INTO users VALUES (1, 'ada@example.com')")
+        val result = exec("INSERT INTO profiles VALUES (1, 'ada@example.com')")
+        assertEquals(InsertResult(1), result)
+        assertFailsWith<KestrelException> {
+            run("INSERT INTO profiles VALUES (2, 'nobody@example.com')")
+        }
+    }
+
+    @Test
+    fun foreignKeyRejectsReferencingAColumnWithoutEnforcedUniqueness() {
+        run("CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT)")
+        val error = assertFailsWith<KestrelException> {
+            run("CREATE TABLE profiles (id INTEGER PRIMARY KEY, user_email TEXT REFERENCES users(email))")
+        }
+        assertTrue(error.message!!.contains("PRIMARY KEY or UNIQUE"), error.message!!)
+    }
+
+    @Test
+    fun cascadeUpdatePropagatesThroughAUniqueReferencedColumn() {
+        run("CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT UNIQUE)")
+        run(
+            "CREATE TABLE profiles (id INTEGER PRIMARY KEY, " +
+                "user_email TEXT REFERENCES users(email) ON UPDATE CASCADE)"
+        )
+        run("INSERT INTO users VALUES (1, 'old@example.com')")
+        run("INSERT INTO profiles VALUES (1, 'old@example.com')")
+        run("UPDATE users SET email = 'new@example.com' WHERE id = 1")
+        val result = select("SELECT user_email FROM profiles WHERE id = 1")
+        assertEquals(listOf(listOf(text("new@example.com"))), result.rows)
+    }
+
+    @Test
+    fun rollbackAfterAFailedInsertLeavesUniqueStateReusable() {
+        run("CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT UNIQUE)")
+        run("BEGIN")
+        run("INSERT INTO users VALUES (1, 'ada@example.com')")
+        run("ROLLBACK")
+        val result = exec("INSERT INTO users VALUES (1, 'ada@example.com')")
+        assertEquals(InsertResult(1), result)
+        assertEquals(listOf(listOf(int(1), text("ada@example.com"))), select("SELECT id, email FROM users").rows)
+    }
+}