Migrating Syntax

-- Generally, the HAVING clause must appear after the GROUP BY clause, but Oracle allows HAVING to appear before or after the GROUP BY clause. Therefore, you need to move the HAVING clause after the GROUP BY clause in the target database.
SELECT
id,
count(*),
sum(remain_users) 
FROM LYC.APEX2_DYNAMIC_ADD_REMAIN_TEST 
HAVING id <= 5
GROUP BY id;

UNIQUE keywords are migrated as DISTINCT keywords.
SELECT UNIQUE add_users FROM LYC.APEX2_DYNAMIC_ADD_REMAIN_TEST;

-- In NVL2(expression,value1,value2), if the expression is not Null, NVL2 returns Value1. If the expression is Null, NVL2 returns Value2.
SELECT NVL2(add_users, 1, 2) FROM LYC.APEX2_DYNAMIC_ADD_REMAIN_TEST SHERE rownum <= 2;