improve sqlparser

src/main/java/com/alibaba/druid/sql/parser/SQLExprParser.java

@@ -319,6 +319,7 @@ public class SQLExprParser extends SQLParser {
             case ESCAPE:
             case OVER:
             case ORDER:
+            case CONSTRAINT:
                 sqlExpr = new SQLIdentifierExpr(lexer.stringVal());
                 lexer.nextToken();
                 break;

src/main/java/com/alibaba/druid/sql/parser/SQLParser.java

@@ -193,6 +193,7 @@ public class SQLParser {
         switch (lexer.token()) {
             case KEY:
             case INTERVAL:
+            case CONSTRAINT:
                 alias = lexer.token().name();
                 lexer.nextToken();
                 return alias;