SQL-02

Question 1

What is an SQL parser?

Answer 1

An SQL parser is a tool or component of a database management system that reads and interprets SQL queries. It breaks down the queries into understandable segments for further processing.

Question 2

What role does an SQL parser play in the execution of an SQL query?

Answer 2

The SQL parser’s primary role is to parse the query, which involves checking syntax, validating SQL commands, and organizing the parts of the query into a structured format for the database to execute.

Question 3

How does an SQL parser analyze the syntax of a query?

Answer 3

The parser performs syntax analysis to ensure the query complies with SQL grammar rules. It checks for proper use of keywords, expressions, operators, and overall query structure.

Question 4

What is the OFFSET clause in SQL?

Answer 4

The OFFSET clause in SQL is used to skip a specified number of rows before starting to return rows from the query. It’s often used with the LIMIT clause for pagination purposes.

Question 5

How is the OFFSET clause typically used in an SQL query?

Answer 5

The basic syntax is SELECT * FROM table LIMIT x OFFSET y;, where x is the number of rows to return, and y is the number of rows to skip.

Question 6

How does using OFFSET affect query performance?

Answer 6

While OFFSET is useful for pagination, it can lead to performance issues on large tables, as the database still reads through all the preceding rows before returning the result.

Question 7

Is there an alternative to using OFFSET in newer SQL versions?

Answer 7

Yes, newer SQL standards and some databases offer the FETCH NEXT syntax, which can be more readable and aligns with the latest SQL standards. For example, SELECT * FROM table LIMIT x FETCH NEXT y ROWS ONLY;.

Question 8

Does the implementation of OFFSET vary across different SQL databases?

Answer 8

Yes, while the basic concept of OFFSET is similar, the exact syntax and capabilities may vary slightly between different SQL database systems like MySQL, PostgreSQL, and SQL Server.

Question 9

Can you perform conditional calculations in SQL?

Answer 9

Yes, use the CASE statement for conditional logic in calculations. For example, SELECT CASE WHEN column1 > 100 THEN column1 * 0.9 ELSE column1 * 1.1 END AS adjusted_value FROM table; adjusts values based on a condition.

Question 10

How important is the order of operations in SQL calculations?

Answer 10

Very important. SQL follows the standard mathematical order of operations (parentheses, exponents, multiplication and division, addition and subtraction). Always use parentheses to ensure the correct order of calculations.

Question 11

How do you perform basic arithmetic operations in an SQL SELECT statement?

Answer 11

You can perform arithmetic operations like addition, subtraction, multiplication, and division directly on column values. For example, SELECT column1 + column2 AS total FROM table; calculates the sum of two columns.

Question 12

Can you use mathematical functions in inline calculations in SQL?

Answer 12

Yes, SQL supports various mathematical functions like ROUND, CEILING, FLOOR, ABS, etc., which can be used in SELECT statements. For example, SELECT ROUND(column1, 2) FROM table; rounds the values in column1 to 2 decimal places.

Question 13

How do you extract a substring from a string in SQL?

Answer 13

Use the SUBSTRING function. The syntax is SUBSTRING(string FROM start FOR length), where start is the starting position and length is the number of characters to extract.

Question 14

How can you change the case of a specific substring within a string in SQL?

Answer 14

Combine SUBSTRING with UPPER or LOWER. For example, SELECT SUBSTRING(UPPER(column_name) FROM 1 FOR 3) FROM table; extracts the first three characters of column_name and converts them to upper case.

Question 15

What should you consider when using string functions with columns that may contain NULL values?

Answer 15

Be aware that string functions will return NULL if the input is NULL. You can use COALESCE or IFNULL to handle NULL values, e.g., SELECT UPPER(COALESCE(column_name, ‘default’)) FROM table;.

Question 16

change case with substring

Answer 16

select * , concat(lower(substring(customer_first_name, 1 ,1)),
upper(substring(customer_first_name, 2))) as first_name
from farmers_market.customer;

Question 17

Do NULL and blank values have different storage space requirements?

Answer 17

Yes. NULL values usually take up less space than blank strings, as they represent the absence of data, whereas blank strings still require storage for the string data type.

Question 18

How do NULL and blank values affect default column values in SQL?

Answer 18

If a column has a default value, inserting NULL (explicitly or by omission) will store NULL, but inserting an empty string will store the empty string, not the default value.

Question 19

What is the difference between inserting NULL and a blank value into a database?

Answer 19

Inserting NULL means you’re inserting a lack of value. Inserting a blank value (‘’) means you’re inserting an empty string, which is a specific value.

Question 20

How are NULL values handled in SQL aggregate functions?

Answer 20

NULL values are generally ignored by aggregate functions like SUM, AVG, COUNT, etc. For instance, COUNT(column) counts only non-NULL values.

Question 21

How do blank values behave in comparisons in SQL?

Answer 21

Blank values are treated like any other string value. For example, column = ‘’ checks if the column contains an empty string.

Question 22

How do NULL values behave in comparisons in SQL?

Answer 22

In SQL, any comparison with NULL (like column = NULL or column != NULL) results in NULL, which is considered false in a WHERE clause. To check for NULL, use IS NULL or IS NOT NULL.

Question 23

What is a blank or empty string in SQL?

Answer 23

A blank or empty string is a string value with no characters, denoted as ‘’. It is a known value, indicating that the string is empty but not missing.

Question 24

What does NULL represent in SQL?

Answer 24

NULL in SQL represents the absence of a value or an unknown value. It’s a marker indicating that the value is missing or not applicable, not even an empty string.

Question 25

What is a correlated subquery in SQL?

Answer 25

A correlated subquery is a subquery that references one or more columns in the outer SQL query. It is evaluated repeatedly, once for each row processed by the outer query.

Question 26

Can you provide a basic example of a correlated subquery?

Answer 26

Sure, consider SELECT * FROM table1 t1 WHERE EXISTS (SELECT * FROM table2 t2 WHERE t1.id = t2.id);. Here, the subquery depends on the outer query’s t1.id.

Question 27

How do correlated subqueries affect performance?

Answer 27

Correlated subqueries can be slower than non-correlated subqueries because they must be evaluated multiple times – once for each row in the outer query.

Question 28

In what situations are correlated subqueries used?

Answer 28

They are often used in cases where we need to compare or evaluate data in the same table or in different tables based on row-by-row comparisons.

Question 29

How are aggregate functions used in correlated subqueries?

Answer 29

An aggregate function in a correlated subquery can operate on values from the outer query. For example, you can select rows from a table where a column value is greater than the average value in the same table.

Question 30

What is the difference between EXISTS and IN clauses in correlated subqueries?

Answer 30

EXISTS checks for the existence of rows returned by the subquery and is true if the subquery returns any row, while IN compares a column against a list of values returned by the subquery.

Question 31

Can correlated subqueries be used in UPDATE statements?

Answer 31

Yes, you can use a correlated subquery in an UPDATE statement to update rows based on values in the same table or another table.

Question 32

How are correlated subqueries used in DELETE statements?

Answer 32

In a DELETE statement, a correlated subquery can identify rows to be deleted based on conditions related to another table or the same table.