--Basic Select

SELECT COUNT(*)
FROM EMPLOYEE
WHERE Dept_Id=1001;

--Select Sample

SELECT *
FROM [TableName]
SAMPLE (%);

Return a percentage of the select result

--Select GROUP BY

SELECT [ColumnName], [AggregateFunction]
FROM [TableName]
GROUP BY [ColumnName];

SELECT D.Dept_Name, AVG(Getdate()-DOB)
FROM EMPLOYEE E
INNER JOIN DEPARTMENT D ON E.DEPT_ID=DEPT_ID
GROUP BY DEPT_ID;

An aggregate function (AVG, COUNT, MAX, MIN, SUM) is mandatory before the FROM

--SELECT HAVING

SELECT [ColumnName],[AggregateFunction]
FROM [TableName]
GROUP BY [ColumnName]
HAVING [AggregateResult] [Condition] [Value];

SELECT D.Dept_Name, AVG(Getdate()-DOB)
FROM EMPLOYEE E
INNER JOIN DEPARTMENT D ON E.DEPT_ID=DEPT_ID
GROUP BY DEPT_ID
HAVING AVG(SYSDATE-DOB)>40;

The HAVING clause must use the aggregate function returned value as condition.