self join interview questions

10. Self Join (Top 50 SQL Interview Quesions)| GeeksforGeeks

5. What is the difference between an SQL INNER JOIN and an SQL LEFT JOIN?

There are some key differences to remember about these commonly used JOIN variants. INNER JOIN returns rows when there is a match in both tables. LEFT JOIN returns all of the rows from the left table and any matching rows from the right table.

Let’s look at a practical example to explore the differences between these clauses. This will help you confidently answer this common SQL JOIN interview question.

Imagine we have two tables:

  • employees – This table contains each employee’s ID, name, and department ID.
  • id employee_name department_id
    1 Homer Simpson 4
    2 Ned Flanders 1
    3 Barney Gumble 5
    4 Clancy Wiggum 3
    5 Moe Syzslak NULL
  • departments – This table contains each department’s ID and name.
  • department_id department_name
    1 Sales
    2 Engineering
    3 Human Resources
    4 Customer Service
    5 Research and Development

    The following SQL code looks for matches between the employees and departments tables based on the department_id column:

    Executing this code will produce the following result:

    id employee_name department_id department_id department_name
    1 Homer Simpson 4 4 Customer Service
    2 Ned Flanders 1 1 Sales
    3 Barney Gumble 5 5 Research and Development
    4 Clancy Wiggum 3 3 Human Resources

    When looking at our result, you will notice our employee, Moe Szyslak, is missing. In our employees table, this employee has no current department_id. Therefore, no match could be found when you try to join the departments table on this column. Thus, the employee is excluded from the result.

    Now, let’s use a LEFT JOIN and see what result that produces. An SQL LEFT JOIN returns all the values from the left table, plus matched values from the right table. If no match could be found, LEFT JOIN returns a NULL value.

    The syntax for our SQL LEFT JOIN clause is as follows:

    The ON keyword works the same as it did for our INNER JOIN example. We are looking for matching values between the department_id column of our employees table and the department_id column of our departments table.

    Here, our employees table will act as the left table because this is the first table we specify.

    The result of executing this SQL query is the following result set:

    id employee_name department_id department_id department_name
    1 Homer Simpson 4 4 Customer Service
    2 Ned Flanders 1 1 Sales
    3 Barney Gumble 5 5 Research and Development
    4 Clancy Wiggum 3 3 Human Resources
    5 Moe Szyslak NULL NULL NULL

    Notice how Moe Szyslak has been included in this result set, even though there is not a matching department_id in the departments table. This is exactly the purpose of the LEFT JOIN clause, to include all the data from our left table, regardless of whether any matches were found.

    7. Write a query that will JOIN these two tables so that all rows from Table 1 are in the result.

    When interviewing for a data analyst or software developer role, you may be asked to complete a technical challenge involving SQL. A common SQL JOIN interview task is writing a query that will join two tables in a certain way. Let’s imagine that you are asked to write a query that will join two tables so that all rows from Table 1 are in the result.

    First, you must understand the concept of right and left tables.

    In the diagram above, Table 1 is the left table and Table 2 is the right table. In other words, the left table comes first in the query; it gets its name from being to the left of the join condition. The right table comes after the JOIN keyword.

    The LEFT JOIN clause selects data starting from the left table. It matches each row from the left table with rows from the right table, based on the condition of the JOIN clause. It returns all the values from the left table, plus matched values from the right table. If no match is found, LEFT JOIN returns a NULL value. This means that if the ON clause matches no records in the right table, the JOIN will still return that row, but with a NULL in each column from the right table.

    For our practical example, we will use the employees and departments tables from our previous example:

    employees – This table contains each employee’s ID, name, and department ID.

    id employee_name department_id
    1 Homer Simpson 4
    2 Ned Flanders 1
    3 Barney Gumble 5
    4 Clancy Wiggum 3
    5 Moe Syzslak NULL

    departments – This table contains each department’s ID and name.

    department_id department_name
    1 Sales
    2 Engineering
    3 Human Resources
    4 Customer Service
    5 Research and Development

    If we want to keep all of the rows from Table 1 (in this case, employees) we must specify this as our left table.

    The syntax for this LEFT JOIN clause is as follows:

    Executing this query yields this result set:

    id employee_name department_id department_id department_name
    1 Homer Simpson 4 4 Customer Service
    2 Ned Flanders 1 1 Sales
    3 Barney Gumble 5 5 Research and Development
    4 Clancy Wiggum 3 3 Human Resources
    5 Moe Szyslak NULL NULL NULL

    Notice that the employee Moe Szyslak has been included in this result set – even though there is no matching department_id in the departments table. This is exactly the purpose of the LEFT JOIN clause, to include all the data from our left table, regardless of whether any matches were found in the right table.

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  • What does UNION do? What is the difference between UNION and UNION ALL?

    UNION merges the contents of two structurally-compatible tables into a single combined table. The difference between UNION and UNION ALL is that UNION will omit duplicate records whereas UNION ALL will include duplicate records.

    It is important to note that the performance of UNION ALL will typically be better than UNION, since UNION requires the server to do the additional work of removing any duplicates. So, in cases where is is certain that there will not be any duplicates, or where having duplicates is not a problem, use of UNION ALL would be recommended for performance reasons.

    List and explain the different types of JOIN clauses supported in ANSI-standard SQL.

    ANSI-standard SQL specifies five types of JOIN clauses as follows:

  • INNER JOIN (a.k.a. “simple join”): Returns all rows for which there is at least one match in BOTH tables. This is the default type of join if no specific JOIN type is specified.
  • LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table, and the matched rows from the right table; i.e., the results will contain all records from the left table, even if the JOIN condition doesn’t find any matching records in the right table. This means that if the ON clause doesn’t match any records in the right table, the JOIN will still return a row in the result for that record in the left table, but with NULL in each column from the right table.
  • RIGHT JOIN (or RIGHT OUTER JOIN): Returns all rows from the right table, and the matched rows from the left table. This is the exact opposite of a LEFT JOIN; i.e., the results will contain all records from the right table, even if the JOIN condition doesn’t find any matching records in the left table. This means that if the ON clause doesn’t match any records in the left table, the JOIN will still return a row in the result for that record in the right table, but with NULL in each column from the left table.
  • FULL JOIN (or FULL OUTER JOIN): Returns all rows for which there is a match in EITHER of the tables. Conceptually, a FULL JOIN combines the effect of applying both a LEFT JOIN and a RIGHT JOIN; i.e., its result set is equivalent to performing a UNION of the results of left and right outer queries.
  • CROSS JOIN: Returns all records where each row from the first table is combined with each row from the second table (i.e., returns the Cartesian product of the sets of rows from the joined tables). Note that a CROSS JOIN can either be specified using the CROSS JOIN syntax (“explicit join notation”) or (b) listing the tables in the FROM clause separated by commas without using a WHERE clause to supply join criteria (“implicit join notation”).
  • Given the following tables:

    What will be the result of the query below?

    Explain your answer and also provide an alternative version of this query that will avoid the issue that it exposes.

    Surprisingly, given the sample data provided, the result of this query will be an empty set. The reason for this is as follows: If the set being evaluated by the SQL NOT IN condition contains any values that are null, then the outer query here will return an empty set, even if there are many runner ids that match winner_ids in the races table.

    Knowing this, a query that avoids this issue would be as follows:

    Note, this is assuming the standard SQL behavior that you get without modifying the default ANSI_NULLS setting.

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    Given two tables created and populated as follows:

    What will the result be from the following query:

    The result of the query will be as follows:

    The EXISTS clause in the above query is a red herring. It will always be true since ID is not a member of dbo.docs. As such, it will refer to the envelope table comparing itself to itself!

    The idnum value of NULL will not be set since the join of NULL will not return a result when attempting a match with any value of envelope.

    Assume a schema of Emp ( Id, Name, DeptId ) , Dept ( Id, Name).

    If there are 10 records in the Emp table and 5 records in the Dept table, how many rows will be displayed in the result of the following SQL query:

    The query will result in 50 rows as a “cartesian product” or “cross join”, which is the default whenever the ‘where’ clause is omitted.

    Given two tables created as follows

    Write a query to fetch values in table test_a that are and not in test_b without using the NOT keyword.

    Note, Oracle does not support the above INSERT syntax, so you would need this instead:

    In SQL Server, PostgreSQL, and SQLite, this can be done using the except keyword as follows:

    In Oracle, the minus keyword is used instead. Note that if there are multiple columns, say ID and Name, the column should be explicitly stated in Oracle queries: Select ID from test_a minus select ID from test_b

    MySQL does not support the except function. However, there is a standard SQL solution that works in all of the above engines, including MySQL:

    Write a SQL query to find the 10th highest employee salary from an Employee table. Explain your answer.

    (Note: You may assume that there are at least 10 records in the Employee table.)

    This can be done as follows:

    This works as follows:

    First, the SELECT DISTINCT TOP (10) Salary FROM Employee ORDER BY Salary DESC query will select the top 10 salaried employees in the table. However, those salaries will be listed in descending order. That was necessary for the first query to work, but now picking the top 1 from that list will give you the highest salary not the the 10th highest salary.

    Therefore, the second query reorders the 10 records in ascending order (which the default sort order) and then selects the top record (which will now be the lowest of those 10 salaries).

    Not all databases support the TOP keyword. For example, MySQL and PostreSQL use the LIMIT keyword, as follows:

    Or even more concisely, in MySQL this can be:

    And in PostgreSQL this can be:

    Write a SQL query using UNION ALL (not UNION) that uses the WHERE clause to eliminate duplicates. Why might you want to do this?

    You can avoid duplicates using UNION ALL and still run much faster than UNION DISTINCT (which is actually same as UNION) by running a query like this:

    The key is the AND a!=X part. This gives you the benefits of the UNION (a.k.a., UNION DISTINCT) command, while avoiding much of its performance hit.

    Given the following tables:

    Write a query to to get the list of users who took the a training lesson more than once in the same day, grouped by user and training lesson, each ordered from the most recent lesson date to oldest date.

    What is an execution plan? When would you use it? How would you view the execution plan?

    An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL server’s query optimizer for a stored procedure or ad hoc query. Execution plans are very useful for helping a developer understand and analyze the performance characteristics of a query or stored procedure, since the plan is used to execute the query or stored procedure.

    In many SQL systems, a textual execution plan can be obtained using a keyword such as EXPLAIN, and visual representations can often be obtained as well. In Microsoft SQL Server, the Query Analyzer has an option called “Show Execution Plan” (located on the Query drop down menu). If this option is turned on, it will display query execution plans in a separate window when a query is run.

    List and explain each of the ACID properties that collectively guarantee that database transactions are processed reliably.

    ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties that guarantee that database transactions are processed reliably. They are defined as follows:

  • Atomicity. Atomicity requires that each transaction be “all or nothing”: if one part of the transaction fails, the entire transaction fails, and the database state is left unchanged. An atomic system must guarantee atomicity in each and every situation, including power failures, errors, and crashes.
  • Consistency. The consistency property ensures that any transaction will bring the database from one valid state to another. Any data written to the database must be valid according to all defined rules, including constraints, cascades, triggers, and any combination thereof.
  • Isolation. The isolation property ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially, i.e., one after the other. Providing isolation is the main goal of concurrency control. Depending on concurrency control method (i.e. if it uses strict – as opposed to relaxed – serializability), the effects of an incomplete transaction might not even be visible to another transaction.
  • Durability. Durability means that once a transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors. In a relational database, for instance, once a group of SQL statements execute, the results need to be stored permanently (even if the database crashes immediately thereafter). To defend against power loss, transactions (or their effects) must be recorded in a non-volatile memory.
  • Given a table dbo.users where the column user_id is a unique numeric identifier, how can you efficiently select the first 100 odd user_id values from the table?

    (Assume the table contains well over 100 records with odd user_id values.)

    SELECT TOP 100 user_id FROM dbo.users WHERE user_id % 2 = 1 ORDER BY user_id

    What are the NVL and the NVL2 functions in SQL? How do they differ?

    Both the NVL(exp1, exp2) and NVL2(exp1, exp2, exp3) functions check the value exp1 to see if it is null.

    With the NVL(exp1, exp2) function, if exp1 is not null, then the value of exp1 is returned; otherwise, the value of exp2 is returned, but case to the same data type as that of exp1.

    With the NVL2(exp1, exp2, exp3) function, if exp1 is not null, then exp2 is returned; otherwise, the value of exp3 is returned.

    How can you select all the even number records from a table? All the odd number records?

    To select all the even number records from a table:

    To select all the odd number records from a table:

    What is the difference between the RANK() and DENSE_RANK() functions? Provide an example.

    The only difference between the RANK() and DENSE_RANK() functions is in cases where there is a “tie”; i.e., in cases where multiple values in a set have the same ranking. In such cases, RANK() will assign non-consecutive “ranks” to the values in the set (resulting in gaps between the integer ranking values when there is a tie), whereas DENSE_RANK() will assign consecutive ranks to the values in the set (so there will be no gaps between the integer ranking values in the case of a tie).

    For example, consider the set {25, 25, 50, 75, 75, 100}. For such a set, RANK() will return {1, 1, 3, 4, 4, 6} (note that the values 2 and 5 are skipped), whereas DENSE_RANK() will return {1,1,2,3,3,4}.

    What is the difference between the WHERE and HAVING clauses?

    When GROUP BY is not used, the WHERE and HAVING clauses are essentially equivalent.

    However, when GROUP BY is used:

  • The WHERE clause is used to filter records from a result. The filtering occurs before any groupings are made.
  • The HAVING clause is used to filter values from a group (i.e., to check conditions after aggregation into groups has been performed).
  • Given a table Employee having columns empName and empId, what will be the result of the SQL query below?

    “Order by 2” is only valid when there are at least two columns being used in select statement. However, in this query, even though the Employee table has 2 columns, the query is only selecting 1 column name, so “Order by 2” will cause the statement to throw an error while executing the above sql query.

    What will be the output of the below query, given an Employee table having 10 records?

    This query will return 10 records as TRUNCATE was executed in the transaction. TRUNCATE does not itself keep a log but BEGIN TRANSACTION keeps track of the TRUNCATE command.

  • What is the difference between single-row functions and multiple-row functions?
  • What is the group by clause used for?
  • Single-row functions work with single row at a time. Multiple-row functions work with data of multiple rows at a time.
  • The group by clause combines all those records that have identical values in a particular field or any group of fields.
  • Imagine a single column in a table that is populated with either a single digit (0-9) or a single character (a-z, A-Z). Write a SQL query to print ‘Fizz’ for a numeric value or ‘Buzz’ for alphabetical value for all values in that column.

    Example:

    [d, x, T, 8, a, 9, 6, 2, V]

    …should output:

    [Buzz, Buzz, Buzz, Fizz, Buzz,Fizz, Fizz, Fizz, Buzz]

    What is the difference between char and varchar2?

    When stored in a database, varchar2 uses only the allocated space. E.g. if you have a varchar2(1999) and put 50 bytes in the table, it will use 52 bytes.

    But when stored in a database, char always uses the maximum length and is blank-padded. E.g. if you have char(1999) and put 50 bytes in the table, it will consume 2000 bytes.

    Write an SQL query to display the text CAPONE as:

    Or in other words, an SQL query to transpose text.

    In Oracle SQL, this can be done as follows:

    Can we insert a row for identity column implicitly?

    Yes, like so:

    Given this table:

    What will be the output of below snippet?

    Table is as follows:

    ID C1 C2 C3
    1 Red Yellow Blue
    2 NULL Red Green
    3 Yellow NULL Violet

    Print the rows which have ‘Yellow’ in one of the columns C1, C2, or C3, but without using OR.

    Write a query to insert/update Col2’s values to look exactly opposite to Col1’s values.

    Col1 Col2
    1 0
    0 1
    0 1
    0 1
    1 0
    0 1
    1 0
    1 0

    Or if the type is numeric:

    How do you get the last id without the max function?

    In MySQL:

    In SQL Server:

    What is the difference between IN and EXISTS?

    IN:

  • Works on List result set
  • Doesn’t work on subqueries resulting in Virtual tables with multiple columns
  • Compares every value in the result list
  • Performance is comparatively SLOW for larger resultset of subquery
  • EXISTS:

  • Works on Virtual tables
  • Is used with co-related queries
  • Exits comparison when match is found
  • Performance is comparatively FAST for larger resultset of subquery
  • Suppose in a table, seven records are there.

    The column is an identity column.

    Now the client wants to insert a record after the identity value 7 with its identity value starting from 10.

    Is it possible? If so, how? If not, why not?

    Yes, it is possible, using a DBCC command:

    How can you use a CTE to return the fifth highest (or Nth highest) salary from a table?

    Given the following table named A:

    Write a single query to calculate the sum of all positive values of x and he sum of all negative values of x.

    Given the table mass_table:

    weight
    5.67
    34.567
    365.253
    34

    Write a query that produces the output:

    weight kg gms
    5.67 5 67
    34.567 34 567
    365.253 365 253
    34 34 0
    Emp_Id Emp_name Salary Manager_Id
    10 Anil 50000 18
    11 Vikas 75000 16
    12 Nisha 40000 18
    13 Nidhi 60000 17
    14 Priya 80000 18
    15 Mohit 45000 18
    16 Rajesh 90000
    17 Raman 55000 16
    18 Santosh 65000 17

    Write a query to generate below output:

    Manager_Id Manager Average_Salary_Under_Manager
    16 Rajesh 65000
    17 Raman 62500
    18 Santosh 53750

    How do you copy data from one table to another table ?

    Find the SQL statement below that is equal to the following: SELECT name FROM customer WHERE state = VA;

  • SELECT name IN customer WHERE state IN (VA);
  • SELECT name IN customer WHERE state = VA;
  • SELECT name IN customer WHERE state = V;
  • SELECT name FROM customer WHERE state IN (VA);
  • SELECT name FROM customer WHERE state IN (VA);
  • Given these contents of the Customers table:

    Here is a query written to return the list of customers not referred by Jane Smith:

    What will be the result of the query? Why? What would be a better way to write it?

    Although there are 4 customers not referred by Jane Smith (including Jane Smith herself), the query will only return one: Pat Richards. All the customers who were referred by nobody at all (and therefore have NULL in their ReferredBy column) don’t show up. But certainly those customers weren’t referred by Jane Smith, and certainly NULL is not equal to 2, so why didn’t they show up?

    SQL Server uses three-valued logic, which can be troublesome for programmers accustomed to the more satisfying two-valued logic (TRUE or FALSE) most programming languages use. In most languages, if you were presented with two predicates: ReferredBy = 2 and ReferredBy <> 2, you would expect one of them to be true and one of them to be false, given the same value of ReferredBy. In SQL Server, however, if ReferredBy is NULL, neither of them are true and neither of them are false. Anything compared to NULL evaluates to the third value in three-valued logic: UNKNOWN.

    The query should be written in one of two ways:

    …or:

    Watch out for the following, though!

    This will return the same faulty set as the original. Why? We already covered that: Anything compared to NULL evaluates to the third value in the three-valued logic: UNKNOWN. That “anything” includes NULL itself! That’s why SQL Server provides the IS NULL and IS NOT NULL operators to specifically check for NULL. Those particular operators will always evaluate to true or false.

    Even if a candidate doesn’t have a great amount of experience with SQL Server, diving into the intricacies of three-valued logic in general can give a good indication of whether they have the ability learn it quickly or whether they will struggle with it.

    Given a table TBL with a field Nmbr that has rows with the following values:

    1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 0, 1, 0, 1, 0, 1

    Write a query to add 2 where Nmbr is 0 and add 3 where Nmbr is 1.

    This can be done as follows:

    Suppose we have a Customer table containing the following data:

    Write a single SQL statement to concatenate all the customer names into the following single semicolon-separated string:

    This is close, but will have an undesired trailing ;. One way of fixing that could be:

    In PostgreSQL one can also use this syntax to achieve the fully correct result:

    How do you get the Nth-highest salary from the Employee table without a subquery or CTE?

    This will give the third-highest salary from the Employee table. Accordingly we can find out Nth salary using LIMIT (N-1),1.

    But MS SQL Server doesn’t support that syntax, so in that case:

    OFFSET’s parameter corresponds to the (N-1) above.

    How to find a duplicate record?

  • duplicate records with one field
  • duplicate records with more than one field
  • duplicate records with one field
     SELECT name, COUNT(email) FROM users GROUP BY email HAVING COUNT(email) > 1 
  • duplicate records with more than one field
     SELECT name, email, COUNT(*) FROM users GROUP BY name, email HAVING COUNT(*) > 1 
  • Considering the database schema displayed in the SQLServer-style diagram below, write a SQL query to return a list of all the invoices. For each invoice, show the Invoice ID, the billing date, the customer’s name, and the name of the customer who referred that customer (if any). The list should be ordered by billing date.

    This question simply tests the candidate’s ability take a plain-English requirement and write a corresponding SQL query. There is nothing tricky in this one, it just covers the basics:

  • Did the candidate remember to use a LEFT JOIN instead of an inner JOIN when joining the customer table for the referring customer name? If not, any invoices by customers not referred by somebody will be left out altogether.
  • Did the candidate alias the tables in the JOIN? Most experienced T-SQL programmers always do this, because repeating the full table name each time it needs to be referenced gets tedious quickly. In this case, the query would actually break if at least the Customer table wasn’t aliased, because it is referenced twice in different contexts (once as the table which contains the name of the invoiced customer, and once as the table which contains the name of the referring customer).
  • Did the candidate disambiguate the Id and Name columns in the SELECT? Again, this is something most experienced programmers do automatically, whether or not there would be a conflict. And again, in this case there would be a conflict, so the query would break if the candidate neglected to do so.
  • Note that this query will not return Invoices that do not have an associated Customer. This may be the correct behavior for most cases (e.g., it is guaranteed that every Invoice is associated with a Customer, or unmatched Invoices are not of interest). However, in order to guarantee that all Invoices are returned no matter what, the Invoices table should be joined with Customers using LEFT JOIN:

    There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every “A” candidate worth hiring will be able to answer them all, nor does answering them all guarantee an “A” candidate. At the end of the day, hiring remains an art, a science — and a lot of work.

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    Submitted questions and answers are subject to review and editing, and may or may not be selected for posting, at the sole discretion of Toptal, LLC.

    Duy has 20+ years of software development experience using Microsofts technology stack, primarily with .NET. He builds high-quality and high-performance back-end systems and creates web applications with good UX using modern frameworks like Angular or React. Duys proactive and results-oriented with a love of not only writing but also removing code to ensure efficient, stable, and flexible software is delivered within the projected scope and budget.

    Matthew has over 15 years of experience in database management and software development, with a strong focus on full-stack web applications. He specializes in Django and Vue.js with expertise deploying to both server and serverless environments on AWS. He also works with relational databases and large datasets.

    Vedansh is a senior software engineer at HackerRank and has 5+ years of experience in writing scalable, maintainable code in Rails and Python. He has built apps that are being used by 9 million+ developers around the world. He strongly believes in TDD and communicates really well and has worked with teams of different sizes across the globe. He also comes with a strong background in freelancing with various startups in the valley.

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    FAQ

    What are the best scenarios to use a self join?

    A self join is a join in which a table is joined with itself (which is also called Unary relationships), especially when the table has a FOREIGN KEY which references its own PRIMARY KEY. To join a table itself means that each row of the table is combined with itself and with every other row of the table.

    Why would you use a self join?

    Answer: The best example of self join in the real world is when we have a table with Employee data and each row contains information about employee and his/her manager. You can use self join in this scenario and retrieve relevant information.

    What is join in SQL interview questions?

    A self join allows you to join a table to itself. It helps query hierarchical data or compare rows within the same table. A self join uses the inner join or left join clause.

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