Convert user ref insertion SQL to platform-agnostic syntax

Model/src/main/java/org/gusdb/wdk/model/user/UserReferenceFactory.java

@@ -30,15 +30,8 @@ class UserReferenceFactory {
   public static final String USER_SCHEMA_MACRO = "$$USER_SCHEMA$$";
   private static final String IS_GUEST_VALUE_MACRO = "$$IS_GUEST$$";
 
-  // SQL and types to insert previously unknown user refs into the users table
-  private static final String INSERT_USER_REF_SQL =
-      "insert" +
-      "  when not exists (select 1 from " + USER_SCHEMA_MACRO + TABLE_USERS + " where " + COL_USER_ID + " = ?)" +
-      "  then" +
-      "  into " + USER_SCHEMA_MACRO + TABLE_USERS + " (" + COL_USER_ID + "," + COL_IS_GUEST + "," + COL_FIRST_ACCESS +")" +
-      "  select ?, " + IS_GUEST_VALUE_MACRO + ", ? from dual";
-
-  private static final Integer[] INSERT_USER_REF_PARAM_TYPES = { Types.BIGINT, Types.BIGINT, Types.TIMESTAMP };
+  // types used by SQL returned by getInsertUserRefSql() below
+  private static final Integer[] INSERT_USER_REF_PARAM_TYPES = { Types.BIGINT, Types.TIMESTAMP };
 
   // SQL and types to select user ref by ID
   private static final String SELECT_USER_REF_BY_ID_SQL =
@@ -80,7 +73,7 @@ public int addUserReference(User user) throws WdkModelException {
       long userId = user.getUserId();
       boolean isGuest = user.isGuest();
       Timestamp insertedOn = new Timestamp(new Date().getTime());
-      String sql = INSERT_USER_REF_SQL
+      String sql = getInsertUserRefSql()
           .replace(USER_SCHEMA_MACRO, _userSchema)
           .replace(IS_GUEST_VALUE_MACRO, _userDb.getPlatform().convertBoolean(isGuest).toString());
       return new SQLRunner(_userDb.getDataSource(), sql, "insert-user-ref")
@@ -91,6 +84,15 @@ public int addUserReference(User user) throws WdkModelException {
     }
   }
 
+  private String getInsertUserRefSql() {
+    return "MERGE INTO " + USER_SCHEMA_MACRO + TABLE_USERS + " tgt " +
+        "USING (SELECT ? AS user_id, ? AS first_access, " + IS_GUEST_VALUE_MACRO + " AS is_guest" + _userDb.getPlatform().getDummyTable() + ") src " +
+        "ON (tgt." + COL_USER_ID + " = src.user_id) " +
+        "WHEN NOT MATCHED THEN " +
+        "INSERT (" + COL_USER_ID + ", " + COL_IS_GUEST + ", " + COL_FIRST_ACCESS + ") " +
+        "VALUES (src.user_id, src.is_guest, src.first_access)";
+  }
+
   // FIXME: see if this is actually needed anywhere?  E.g. do we ever need to look up user refs by user ID to find last login?
   public Optional<UserReference> getUserReference(long userId) throws WdkModelException {
     try {