SQL Basic Interview questions

Question: What type of joins have you used?

Answer: Joins knowledge is MUST HAVE. This interview question is quite nice because most people used inner join and (left/right) outer join which is rather mandatory knowledge but those more experienced will also mention cross join and self-join. In SQL Server you can also get full outer join.

Question: How can you combine two tables/views together? For instance one table contains 100 rows and the other one contains 200 rows, have exactly the same fields and you want to show a query with all data (300 rows). This sql interview question can get complicated.

Answer: You use UNION operator. You can drill down this question and ask what is the different between UNION and UNION ALL (the first one removes duplicates (not always desirable)… in other words shows only DISTINCT rows….Union ALL just combines so it is also faster). More tricky question are how to sort the view (you use order by at the last query), how to name fields so they appear in query results/view schema (first query field names are used). How to filter groups when you use union using SQL (you would create separate query or use common table expression (CTE) or use unions in from with ().

Question: What is the difference between where and having clause?

Answer: in SQL Where filters data on lowest row level. Having filters data after group by has been performed so it filters on "groups"

Question: How would apply date range filter?

Answer: This is tricky question. You can use simple condition >= and <= or similar or use between/and but the trick is to know your exact data type. Sometimes date fields contain time and that is where the query can go wrong so it is recommended to use some date related functions to remove the time issue. In SQL Server common function to do that is datediff function. You also have to be aware of different time zones and server time zone.

Question: What type of wildcards have you used? This is usually one of mandatory sql interview question.

Answer: First question is what is a wildcard? Wildcards are special characters that allow matching string without having exact match. In simple word they work like contains or begins with. Wildcard characters are software specific and in SQL Server we have % which represent any groups of characters, _ that represent one character (any) and you also get [] where we can [ab] which means characters with letter a or b in a specific place.