fix: replace partition key representations consistently

src/main/java/net/sf/jsqlparser/statement/create/table/TablePartitioning.java

@@ -43,10 +43,10 @@ public List<Index.ColumnParams> getKeyColumns() {
     }
 
     public void setKeyColumns(List<Index.ColumnParams> keyColumns) {
+        if (keyColumns != null) {
+            clearKeyRepresentation();
+        }
         this.keyColumns = keyColumns;
-        expression = null;
-        expressionList = null;
-        columns = null;
     }
 
     private Type type;
@@ -97,8 +97,10 @@ public Expression getExpression() {
     }
 
     public void setExpression(Expression expression) {
+        if (expression != null) {
+            clearKeyRepresentation();
+        }
         this.expression = expression;
-        keyColumns = null;
     }
 
     /**
@@ -111,17 +113,29 @@ public ExpressionList<Expression> getExpressionList() {
     }
 
     public void setExpressionList(ExpressionList<Expression> expressionList) {
+        if (expressionList != null) {
+            clearKeyRepresentation();
+        }
         this.expressionList = expressionList;
-        keyColumns = null;
     }
 
     public ExpressionList<Column> getColumns() {
         return columns;
     }
 
     public void setColumns(ExpressionList<Column> columns) {
+        if (columns != null) {
+            clearKeyRepresentation();
+        }
         this.columns = columns;
+    }
+
+    /** Selecting a key representation must replace the previously rendered key. */
+    private void clearKeyRepresentation() {
         keyColumns = null;
+        expression = null;
+        expressionList = null;
+        columns = null;
     }
 
     public Integer getAlgorithm() {

src/test/java/net/sf/jsqlparser/statement/create/PartitionKeyMutationTest.java

@@ -0,0 +1,89 @@
+/*-
+ * #%L
+ * JSQLParser library
+ * %%
+ * Copyright (C) 2004 - 2026 JSQLParser
+ * %%
+ * Dual licensed under GNU LGPL 2.1 or Apache License 2.0
+ * #L%
+ */
+package net.sf.jsqlparser.statement.create;
+
+import static org.junit.jupiter.api.Assertions.*;
+import java.util.List;
+import net.sf.jsqlparser.JSQLParserException;
+import net.sf.jsqlparser.expression.Expression;
+import net.sf.jsqlparser.expression.operators.relational.ExpressionList;
+import net.sf.jsqlparser.parser.AbstractJSqlParser.Dialect;
+import net.sf.jsqlparser.parser.CCJSqlParserUtil;
+import net.sf.jsqlparser.schema.Column;
+import net.sf.jsqlparser.statement.create.table.CreateTable;
+import net.sf.jsqlparser.statement.create.table.Index;
+import net.sf.jsqlparser.statement.create.table.TablePartitioning;
+import net.sf.jsqlparser.util.deparser.StatementDeParser;
+import org.junit.jupiter.api.Test;
+
+class PartitionKeyMutationTest {
+    @Test
+    void replacingSingleKeyWithSeveralKeysChangesBothRenderers() throws JSQLParserException {
+        CreateTable table = parse("CREATE TABLE t(id INT,other INT) PARTITION BY RANGE(id)",
+                Dialect.POSTGRESQL);
+        table.getPartitioning().setExpressionList(
+                new ExpressionList<Expression>(new Column("other"), new Column("id")));
+        assertRoundTrip(table,
+                "CREATE TABLE t (id INT, other INT) PARTITION BY RANGE (other, id)",
+                Dialect.POSTGRESQL);
+        assertNull(table.getPartitioning().getExpression());
+    }
+
+    @Test
+    void switchingHashToKeyUsesNewColumnNames() throws JSQLParserException {
+        CreateTable table = parse("CREATE TABLE t(id INT,other INT) PARTITION BY HASH(id)",
+                Dialect.MYSQL);
+        TablePartitioning partition = table.getPartitioning();
+        partition.setType(TablePartitioning.Type.KEY);
+        partition.setColumns(new ExpressionList<>(new Column("other")));
+        assertRoundTrip(table, "CREATE TABLE t (id INT, other INT) PARTITION BY KEY (other)",
+                Dialect.MYSQL);
+        partition.setType(TablePartitioning.Type.HASH);
+        partition.setExpression(new Column("id"));
+        assertNull(partition.getColumns());
+        assertRoundTrip(table, "CREATE TABLE t (id INT, other INT) PARTITION BY HASH (id)",
+                Dialect.MYSQL);
+    }
+
+    @Test
+    void attributedKeysReplaceAndAreReplacedByPlainKeys() throws JSQLParserException {
+        CreateTable table = parse("CREATE TABLE t(id INT,other INT) PARTITION BY RANGE(id,other)",
+                Dialect.POSTGRESQL);
+        TablePartitioning partition = table.getPartitioning();
+        partition.setKeyColumns(
+                List.of(new Index.ColumnParams("other").withOperatorClass("int4_ops")));
+        assertNull(partition.getExpressionList());
+        assertRoundTrip(table,
+                "CREATE TABLE t (id INT, other INT) PARTITION BY RANGE (other int4_ops)",
+                Dialect.POSTGRESQL);
+        partition.setExpression(new Column("id"));
+        assertNull(partition.getKeyColumns());
+        assertRoundTrip(table, "CREATE TABLE t (id INT, other INT) PARTITION BY RANGE (id)",
+                Dialect.POSTGRESQL);
+        partition.setColumns(null);
+        partition.setExpressionList(null);
+        partition.setKeyColumns(null);
+        assertRoundTrip(table, "CREATE TABLE t (id INT, other INT) PARTITION BY RANGE (id)",
+                Dialect.POSTGRESQL);
+    }
+
+    private static CreateTable parse(String sql, Dialect dialect) throws JSQLParserException {
+        return (CreateTable) CCJSqlParserUtil.parse(sql, p -> p.withDialect(dialect));
+    }
+
+    private static void assertRoundTrip(CreateTable table, String expected, Dialect dialect)
+            throws JSQLParserException {
+        assertEquals(expected, table.toString());
+        StringBuilder output = new StringBuilder();
+        table.accept(new StatementDeParser(output), null);
+        assertEquals(expected, output.toString());
+        assertEquals(expected, parse(output.toString(), dialect).toString());
+    }
+}