Are you preparing for a SQL developer interview?
Then read on as we give you the 7 crucial concepts in SQL that you must know thoroughly to help you sail through the interview.
Getting to know SQL
Structured Query Language or SQL is one of the most common languages for organizing and extracting data that is stored in relational databases. This language is a mainstay for most of the people working with data since most of the databases are managed relationally, thus making this language indispensable. SQL is used by data analysts to query tables of data and derive insights from it. It is used by data scientists to load data into their models. Similarly, data engineers and database administrators use SQL to ensure that everyone in their organization has easy and intuitive access to the data they need.
Interviews always depend on your knowledge and experience. However, there are some important concepts in SQL which you must cover. These topics will help you in basic as well as advanced SQL interview questions.
Tables are one of the most basic concepts in SQL that you need to understand as you may find SQL interview questions based on this. For instance- What do you mean by tables in SQL? This is one of the most common question asked by the interviewers, hence knowing what SQL table means is important. A table is a unique set of data with a consistent number of columns or typed data attributes. Each table should have a primary key i.e. a column that uniquely identifies a row.
SQL interview questions based on relationships in SQL are also frequently asked. For instance- What are relationships? Relationships are the links or relations between entities that have something to do with each other. So when two tables are joined, one is always considered as the ‘parent’ in the relationship and the other one as a ‘child’. Relationships and tables are the basic knowledge of SQL that an aspiring SQL developer should have.
Once you understand the basic knowledge of SQL tables and relations, you will be ready to build an understanding of what relationship means. To begin with, you need to understand the modality or the Ordinality of the relationships which specify whether relationship from the parent table to the child table is mandatory or optional.
The next important concept is cardinality or multiplicity of relationships. The SQL interview questions are often based around relationships in SQL to understand if the candidate has the basic knowledge of SQL. Cardinality is either one-to-one or one-to-many or many-to-many.
Both Ordinality and cardinality only scratch the surface of a database structure. Once you are clear with these concepts you can move on to more advanced concepts in SQL such as normalization and identifying relationships. For instance – one of the frequently asked SQL interview questions is – What is normalization and what are the advantages of it? Or explain the different types of normalization?
The concept of index also needs to be learnt thoroughly as one or two SQL interview questions are often based on this. For instance- What is an Index? Or explain the different types of index?
An index is a performance tuning method that allows faster retrieval of records from the table. It basically creates an entry for each value thus making it faster to retrieve data.
DROP, DELETE and TRUNCATE statements
One of the top SQL interview questions is- Explain the difference between TRUNCATE and DELETE statements? Or what is TRUNCATE, DROP and DELETE statements?
DELETE is a Data Manipulation Language or DML command whereas TRUNCATE is a Data Definition Language or DDL command. DELETE statement is used to delete rows from a table whereas to delete all rows from the table and to make the space free, TRUNCATE command is used. The DROP command is used to remove an object from the database.
To understand this concept you should know different subsets of SQL. This is explained in various SQL tutorials that are available online.
Query and Subquery
A query is a request for information or data from a database table or combination of tables whereas subquery is a query within another query.
One should know the concept of query and subquery in SQL very well as it is one of the frequently asked SQL interview questions. For instance, from a simple question like- What is a query? Or what is a subquery? What are its types? To questions which may ask you to write an SQL query for a given data can be asked in an interview.
Hope you will find the above information useful.
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