Imagine you have a sequence of 3 numbers 1,17,8,.. what is the next number ?
To answer questions like that you should generalize a formula to solve any kind of 3 number sequences. Let's assume that the three numbers form a linear equation of a straight line where you have 2 points (1,17) and (17,8). Here I assumed a linear relation where ,it is not forbidden here and applicable, and the relation between each number and the previous one is the same, Now you have 2 points where you can easily a straight line equation of y=mx+b by solving the two equations in m and b 17=m+b and 8=17m+b solve for m and b then you have straight line equation where you can substitute with the same method by 8 to get y or the next number y=8m+b (Solved!)
What is the maximum number of inner sub-queries you can use in SQL ?
In SQL there is no maximum number for inner or subqueries. You can have a subquery inside a subquery inside a subquery and so forth. For Example: SELECT NAME_B FROM ( SELECT NAME AS NAME_B FROM (SELECT NAME FROM EMPLOYEE) A) B; The above query although it is doable but it is not the best engineering practice as with more records in a table you can face performance issues and you should always check the explain plan prior running or committing an SQL script.
What does ETL stands for and give an example for a tool you can use to do ETL?
ETL stands for Extract-Transform-Load, there is also another model which is the ELT or Extract-Load-Transform. The ETL model is the most used. There are several tools that can help do this process from many vendors, one of the most popular tools is the IBM - InfoSphere DataStage. To help imagine what is ETL, assume you have lots of food everywhere and you have an afternoon party that you have to prepare food for. So you take the needed ingredits from everywhere (Extract) and then put it the kitchen where you can use the oven, add some sauces, etc (Transform) and then you will put the final deliverable food on the table (Load).