All basic Core SQL, such as DDL: CREATE, ALTER, DROP, TRUNCATE
DML: INSERT, UPDATE, DELETE
DQL: SELECT
WHERE, ORDER BY, DISTINCT
LIMIT / OFFSET
Aggregate Functions (COUNT, SUM, AVG, MIN, MAX)
GROUP BY & HAVING
JOINS: INNER, LEFT, RIGHT, FULL
Subqueries (Scalar, Correlated, Nested)
UNION, UNION ALL
NULL handling (IS NULL, IFNULL / COALESCE)
Indexes
Views
SQL Keys and some advanced such as Create function, create store procedure, performance tuning, Difference between function Vs Store procedure, 2nd Highest salary,
department highest
SQL Datatype and constraint management
According to your answered they raised questions difficulties
