Difference between GETDATE() vs SYSDATETIME() vs GETUTCDATE() in SQL Server

One of the common question on Microsoft SQL Server interview is, what is the difference between GETDATE(), SYSDATETIME(), and GETUTCDATE(). Even though all three SQL Sever function returns the current date time in SQL Server, there are some subtle differences between them. The main difference between GETDATE() and SYSDATETIME() is that GETDATE returns current date and time as DATETIME but SYSDATETIME returns a DATETIME2 value, which is more precise. The difference between GETDATE() and GETUTCDATE() is in timezone, the GETDATE() function return current date and time in the local timezone, the timezone where your database server is running, but GETUTCDATE() return current time and date in UTC (Universal Time Coordinate) or GMT timezone.

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How to Compare Date in SQL Server Query? Finding All Rows Between Two Dates

It's tricky to use dates in SQL server query, especially if you don't have good knowledge of how DateTime type works in SQL server. For example, one of the frequently asked SQL queries on the interview is to "select all rows where the date is 20151007?" How would you do that? Does following SQL Query will work correctly

select * from table where date = '20151007'

It may or may not, it entirely depends on upon data in your table. When you only provide date part of a DateTime variable, it uses '00:00:00.000' for time part.

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Difference between row_number(), rank() and dense_rank() in SQL Server, Oracle.

Though all three are ranking functions in SQL, also known as window function in Microsoft SQL Server, the difference between rank(), dense_rank(), and row_number() comes when you have ties on ranking i.e. duplicate records. For example, if you are ranking employees by their salaries then what would be the rank of two employees of same salaries? It depends on upon which ranking function you are using e.g. row_number, rank, or dense_rank. The row_number() function always generates a unique ranking even with duplicate records i.e. if the ORDER BY clause cannot distinguish between two rows, it will still give them different rankings, though which record will come earlier or later is decided randomly e.g. in our example two employees Shane and Rick have the same salary and has row number 4 and 5, this is random, if you run again, Shane might come 5th.

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How to remove duplicate rows from a table in SQL

There are a couple of ways to remove duplicate rows from a table in SQL e.g. you can use a temp tables or a window function like row_number() to generate artificial ranking and remove the duplicates. By using a temp table, you can first copy all unique records into a temp table and then delete all data from the original table and then copy unique records again to the original table. This way, all duplicate rows will be removed, but with large tables, this solution will require additional space of the same magnitude of the original table. The second approach doesn't require extra space as it removes duplicate rows directly from the table. It uses a ranking function like row_number() to assign a row number to each row.

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4 Ways to find Nth highest salary in SQL - Oracle, MSSQL and MySQL

One of the most common SQL interview questions is to find the Nth highest salary of employee, where N could be 2, 3, 4 or anything e.g. find the second highest salary in SQL. Sometimes this question is also twisted as to find the nth minimum salary in SQL. Since many Programmers only know the easy way to solve this problem e.g. by using SQL IN clause, which doesn't scale well, they struggle to write the SQL query when Interviewer keep asking about 4th highest, 5th highest and so on. In order to solve this problem effectively, you need to know about some key concepts like correlated subquery, window functions like ROW_NUMER(), RANK() and DENSE_RANK() etc. Once you know the generic logic to solve this problem, you can tackle all those variations by yourself.

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