fix: preserve interval type compatibility

Signed-off-by: 付典 <fudianchn@gmail.com>

Author: fudianchn

src/main/java/net/sf/jsqlparser/expression/IntervalExpression.java

@@ -49,20 +49,32 @@ public void setParameter(String parameter) {
         this.parameter = parameter;
     }
 
+    /**
+     * Returns the interval type for backwards compatibility. If a structured qualifier is present,
+     * its SQL representation is returned.
+     */
     public String getIntervalType() {
-        return intervalType;
+        return intervalQualifier != null ? intervalQualifier.toString() : intervalType;
     }
 
+    /**
+     * Sets the legacy interval type and clears any structured qualifier.
+     */
     public void setIntervalType(String intervalType) {
         this.intervalType = intervalType;
+        this.intervalQualifier = null;
     }
 
     public IntervalQualifier getIntervalQualifier() {
         return intervalQualifier;
     }
 
+    /**
+     * Sets the structured interval qualifier and clears the legacy interval type.
+     */
     public void setIntervalQualifier(IntervalQualifier intervalQualifier) {
         this.intervalQualifier = intervalQualifier;
+        this.intervalType = null;
     }
 
     public Expression getExpression() {

src/test/java/net/sf/jsqlparser/expression/IntervalExpressionTest.java

@@ -62,17 +62,60 @@ void testIntervalSecondWithLeadingAndFractionalPrecisionRoundTrip() throws JSQLP
         TestUtils.assertSqlCanBeParsedAndDeparsed("SELECT INTERVAL '30.12345' SECOND(2, 4)", true);
     }
 
+    @Test
+    void testIntervalSingleFieldKeepsLegacyGetter() throws JSQLParserException {
+        String sql = "SELECT INTERVAL '1' DAY";
+        Select select = (Select) TestUtils.assertSqlCanBeParsedAndDeparsed(sql, true);
+        IntervalExpression interval = (IntervalExpression) select.getPlainSelect()
+                .getSelectItems().get(0).getExpression();
+
+        assertEquals("DAY", interval.getIntervalType());
+        assertEquals("INTERVAL '1' DAY", interval.toString());
+        TestUtils.assertExpressionCanBeDeparsedAs(interval, "INTERVAL '1' DAY");
+    }
+
     @Test
     void testIntervalQualifierStructurallyAttached() throws JSQLParserException {
-        Select select = (Select) CCJSqlParserUtil.parse("SELECT INTERVAL '1' HOUR TO MINUTE");
+        String sql = "SELECT INTERVAL '1' HOUR TO MINUTE";
+        Select select = (Select) TestUtils.assertSqlCanBeParsedAndDeparsed(sql, true);
         IntervalExpression interval = (IntervalExpression) select.getPlainSelect()
                 .getSelectItems().get(0).getExpression();
-        assertNull(interval.getIntervalType(), "single-string intervalType must stay null");
+        assertEquals("HOUR TO MINUTE", interval.getIntervalType());
         IntervalQualifier qualifier = interval.getIntervalQualifier();
         assertNotNull(qualifier, "qualifier must be a structured property of the interval");
         assertEquals("HOUR", qualifier.getLeadingField());
         assertEquals("MINUTE", qualifier.getTrailingField());
         assertEquals("HOUR TO MINUTE", qualifier.toString());
+        TestUtils.assertExpressionCanBeDeparsedAs(interval, "INTERVAL '1' HOUR TO MINUTE");
+    }
+
+    @Test
+    void testLegacySetterOverridesStructuredQualifier() throws JSQLParserException {
+        Select select = (Select) CCJSqlParserUtil.parse("SELECT INTERVAL '1' DAY");
+        IntervalExpression interval = (IntervalExpression) select.getPlainSelect()
+                .getSelectItems().get(0).getExpression();
+
+        interval.setIntervalType("HOUR");
+
+        assertNull(interval.getIntervalQualifier());
+        assertEquals("HOUR", interval.getIntervalType());
+        assertEquals("INTERVAL '1' HOUR", interval.toString());
+        TestUtils.assertExpressionCanBeDeparsedAs(interval, "INTERVAL '1' HOUR");
+        TestUtils.assertSqlCanBeParsedAndDeparsed("SELECT " + interval, true);
+    }
+
+    @Test
+    void testStructuredSetterOverridesLegacyType() throws JSQLParserException {
+        IntervalExpression interval = new IntervalExpression()
+                .withExpression(new StringValue("1"))
+                .withIntervalType("DAY");
+
+        interval.withIntervalQualifier(new IntervalQualifier("HOUR", null, "MINUTE", null));
+
+        assertEquals("HOUR TO MINUTE", interval.getIntervalType());
+        assertEquals("INTERVAL '1' HOUR TO MINUTE", interval.toString());
+        TestUtils.assertExpressionCanBeDeparsedAs(interval, "INTERVAL '1' HOUR TO MINUTE");
+        TestUtils.assertSqlCanBeParsedAndDeparsed("SELECT " + interval, true);
     }
 
     @Test