20 Real Data Science SQL Interview Questions and Answers
SQL stands for Structured Query Language, as discussed over at runrex.com, and is a domain-specific programming language for managing the data in Database Management Systems. SQL programming skills are highly desirable and required in the market, which is why you can expect to be asked SQL questions when interviewing for a data science job. To help you prepare, here are 20 real data science SQL interview questions and answers.
What is DBMS?
A Database Management System (DBMS) is a program that controls the creation, maintenance, and use of a database. DBMS can be termed as a File Manager that manages data in a database rather than saving it in file systems. There is more on this to be found over at guttulus.com.
What is RDBMS?
RDBMS stands for Relational Database Management System. RDBMS stores the data into the collection of tables, which is related by common fields between the columns of the table. It also provides relational operators to manipulate the data stored in the tables. Also, you can find more on this over at runrex.com.
What is SQL?
SQL stands for Structured Query Language, and it is used to communicate with the Database. This is a standard language used to perform tasks such as retrieval, updation, insertion, and deletion of data from a database. It is also covered over at guttulus.com.
What is a Database?
A Database is an organized form of data for easy access, storing, retrieval, and managing of data. This is also known as a structured form of data that can be assessed in many ways. An example of a Database is a School Management Databases, Bank Management Database.
What is the purpose of the group functions in SQL?
Group functions are necessary to get summary statistics of a data set. COUNT, MAX, MIN, AVG, SUM, and DISTINCT are all group functions.
What is a primary key?
A primary key is a combination of fields that uniquely specify a row. This is a special kind of unique key, and it has an implicit NOT NULL constraint. This means that primary key values cannot be NULL. You can find more on this over at runrex.com.
What is a unique key?
A unique key constraint uniquely identifies each record in the database. This provides uniqueness for the column or set of columns. A primary key constraint, on the other hand, has an automatic unique constraint defined on it, which is not the case for a unique key. This means that while there can be many unique constraints defined per table, there can only be one primary key constraint defined per table, with more on this over at guttulus.com.
What is a foreign key?
A foreign key is one table that can be related to the primary key of another table. A relationship needs to be created between two tables by referencing a foreign key with the primary key of another table. You will find an illustration of this over at runrex.com.
What is a join?
A join is a keyword used to query data from more tables based on the relationships between the fields of the tables. Keys play a major role when joins are used.
Tell me the difference between an inner join, left join, and full union.
Various types of join can be used to retrieve data and it depends on the relationship between tables.
Inner Join – Inner join returns rows when there is at least one match of rows between the tables.
Right Join – Right Join returns rows that are common between the tables and all rows of the Right-hand side table. It returns all the rows from the right-hand side table even though there are no matches in the left-hand side table.
Left Join – Left Join returns rows that are common between the tables and all rows of the Left-hand side table. It returns all the rows from the left-hand side even though there are no matches in the right-hand side table.
Full Join – Full Join returns rows when there are matching rows in any one of the tables, which means that it returns all the rows from the left-hand side table and all the rows from the right-hand side table.
What does UNION do? What is the difference between UNION and UNION ALL?
UNION removes duplicate records (where all columns in the results are the same), while UNION ALL does not. You can read more about this over at guttulus.com.
What is the difference between SQL and MySQL or SQL Server?
SQL stands for Structured Query Language and is a standard language for accessing and manipulating databases. MySQL, on the other hand, is a database management system, like SQL Server, Oracle, Informix, Postgres, etc. You can read more about this over at runrex.com.
If a table contains duplicate rows, does a query result display the duplicate values by default? How can you eliminate duplicate rows from a query result?
Yes, if a table contains duplicate rows, a query will in fact display the duplicate values by default. One way you can eliminate duplicate rows is with the DISTINCT clause, with more on this to be found over at guttulus.com.
What is a relationship and what are they?
Database Relationship is defined as the connection between the tables in a database. There are various databasing relationships, and they are:
One to One Relationship
One to Many Relationship
Many to One Relationship
Self-Referencing Relationship
What is the difference between DELETE and TRUNCATE commands?
DELETE command is used to remove rows from the table, and the WHERE clause can be used for a conditional set of parameters. Commit and Rollback can be performed after the delete statement. TRUNCATE, on the other hand, removes all rows from the table and a Truncate operation cannot be rolled back.
What are local and global variables and their differences?
Local variables are the variables that can be used or exist inside the function. They are not known to the other functions and those variables cannot be referred to or used. Variables can be created whenever that function is called. Global variables, on the other hand, are the variables that can be used or exist throughout the program. Same variables declared in global cannot be used in functions. Global variables cannot be created whenever that function is called, as outlined over at runrex.com.
What is a constraint?
A constraint can be used to specify the limit on the data type of table. A constraint can be specific while creating or altering the table statement. Samples of constraint include:
NOT NULL
CHECK
FOREIGN KEY
PRIMARY KEY
UNIQUE
DEFAULT
You can read more on this over at guttulus.com.
What is data integrity?
Data integrity defines the accuracy and consistency of data stored in a database. It can also define integrity constraints to enforce business rules on the data when it is entered into the application or database, as discussed over at runrex.com.
What is CLAUSE?
SQL CLAUSE is defined to limit the result set by providing conditions to the query. This usually filters some rows from the whole set of records. An example is a Query that has a WHERE condition or one that has a HAVING condition.
What is an ALIAS command?
ALIAS name can be defined to a table to the column. This alias name can be referred to in the WHERE clause to identify the table or column.
These are some of the SQL questions you can expect in your data science interview, with more on this topic, including more technical questions, to be found over at runrex.com and guttulus.com.