bug fixed for mysql parser. support syntax 'set xxx = ON

src/main/java/com/alibaba/druid/sql/dialect/mysql/parser/MySqlExprParser.java

@@ -695,7 +695,12 @@ public class MySqlExprParser extends SQLExprParser {
             }
         }
 
-        item.setValue(this.expr());
+        if (lexer.token() == Token.ON) {
+            lexer.nextToken();
+            item.setValue(new SQLIdentifierExpr("ON"));
+        } else {
+            item.setValue(this.expr());
+        }
 
         item.setTarget(var);
         return item;

src/main/java/com/alibaba/druid/sql/visitor/SchemaStatVisitor.java

@@ -113,6 +113,7 @@ import com.alibaba.druid.sql.ast.statement.SQLSelectOrderByItem;
 import com.alibaba.druid.sql.ast.statement.SQLSelectQuery;
 import com.alibaba.druid.sql.ast.statement.SQLSelectQueryBlock;
 import com.alibaba.druid.sql.ast.statement.SQLSelectStatement;
+import com.alibaba.druid.sql.ast.statement.SQLSetStatement;
 import com.alibaba.druid.sql.ast.statement.SQLShowTablesStatement;
 import com.alibaba.druid.sql.ast.statement.SQLSubqueryTableSource;
 import com.alibaba.druid.sql.ast.statement.SQLTableElement;
@@ -1729,4 +1730,10 @@ public class SchemaStatVisitor extends SQLASTVisitorAdapter {
 
         return false;
     }
+    
+    @Override
+    public boolean visit(SQLSetStatement x) {
+        return false;
+    }
+
 }