Advanced SQL interview questions and answers for 2022

With the help of Indexes, information retrieval from the database happens faster and with greater efficiency. Thus, indexes improve performance. There are three types of indexes:

• Clustered: Used for reordering tables and searching information with key values.
• Non-clustered: Used for maintaining the order of the tables.
• Unique: They ban fields from having duplicate values.

There can be many non-clustered indexes in a table, however, there can be only one clustered index.

As the name suggests, a synonym is used to give different names to the same object in the database. In the case of object-renaming or object schema-change, existing applications can continue to use older names because of synonyms. A synonym must only reference an object and not another synonym. Additionally, synonyms can also be used to reference objects in different databases or servers, by using 3 or 4 part object names. There can be many names for a single database object as long as all the names directly refer to the same database object.

You must ensure that you know answers to such SQL interview questions as answering them correctly will give you the much-needed confidence for the more difficult ones.

Below are some advantages of using Synonyms:

• Synonyms create a layer of abstraction for the specific object
• For objects, with complex 3 or 4 part names, residing on the same server, Synonyms can give a simpler alias
• Offers the flexibility to change object location without having to change the existing code
• When the name of an object is changed or dropped, Synonym offers backward compatibility for older applications
• Synonyms are also useful in front-end query tools such as Access linked tables and spreadsheets if there is a direct link to these tables

Yes, there are some disadvantages.

• Synonyms are only loosely linked to the referenced object and thus, can be deleted without warning when being used to reference a different database object
• Inside chaining cannot take place, meaning that the synonym of a synonym cannot be created
• One cannot create a table with the same Synonym name
• The checking for the object for which the Synonym is created happens at runtime and not at the time of creation. This means if there is an error, such as a spelling error, it will only show up at runtime creating a problem in accessing the object
• Synonyms cannot be referenced in DDL statements

For SQL interview questions that ask you to talk about the advantages or disadvantages of a certain component or tool, ensure that you list as many as you can. Also, you can make your answer to such an SQL interview question meaty by adding personal anecdotes about some of the advantages or disadvantages.

No. NULL values show an absence of characters, whereas zero is a numerical value. NULL values occur when a character is unavailable or not known. NULL values should also not be confused with blank space because a blank space is not supposed to have any data attached to it, whereas a NULL value shows a data record without any value assigned to it.

A Scalar subquery is when a query returns just one row and one column of data. A Correlated subquery occurs when a query cannot process without information from an outer query. In such cases, table aliases define the scope of the argument and the subquery is parameterized by an outer query. Thus, there is a correlation between the inner and outer queries. As a result, back and forth execution takes place where a single row of results from the outer query passes parameters to the inner query.

SQL interview questions like the one above try to ascertain the depth of your knowledge of SQL.

The function NVL (exp1, exp2) is a conversion function that changes exp1 into the target exp2 under the condition that exp1 is NULL. The data type of exp1 is the same as that of a return value. The function NVL2 (exp1, exp2, exp3), on the other hand, is a checking function, which determines whether exp1 is null or not. When exp1 is not null, exp2 is returned as the result. When exp1 is null, exp3 is returned as the result.

With the auto-increment command, one can generate unique numbers when new records are added to a table. This function is especially useful when one wants to automatically generate the primary key field values upon inserting new records. This command comes in handy on several platforms. The auto-increment command for the SQL servers is “identity”.

The main use of the recursive stored procedure is to make the code calls till the time certain boundary conditions are reached. This helps programmers enhance productivity by using the same code multiple times.

An SQL interview question like this one shows that even though some of the advanced concepts may be easy to understand, they may be difficult to recount when suddenly faced with the question. Thus, when you prepare for SQL interview questions, ensure to revise all types of concepts.

A ‘datawarehouse’ is a system used for analyzing and reporting data. It is very similar to a physical warehouse where inventory is stored and assessed before being sent to a customer. Here, data is stored, analyzed, and reported. A datawarehouse functions as a central repository of data integrated from different areas and sources and makes this data available for use.