SQL SELECT query retrieving data example with database illustration.

6.How do you limit the number of rows returned in a result set?

To limit the number of rows returned in a result set, different database systems have their own syntax. In Oracle ROWNUM is used to limit the number of rows. however, a crucial point to remember is that ROWNUM applies before any sorting in the query, so if sorting is needed, it's best to surround it with a subquery. In MySQL and PostgreSQL the LIMIT clause is used for specifying the maximum number of rows to be returned and it can also combine with OFFSET in order to skip a certain number of rows. therefore, it is the best tool for application pagination. SQL Server uses the keyword TOP usually written immediately after the SELECT keyword for limiting the number of the returned rows like LIMIT does in the other databases. All of these strategies remove all the unnecessary rows in a result set in order to improve performance concentrating data output that is based on the needs of a query.

7.How to sort a result set in ascending or descending order?

To sort a result set, you can use the ORDER BY clause in SQL. You can define the column by using which you want to sort your results. You can also define the sorting order as either ASC for ascending or DESC for descending. For example, SELECT column_name FROM table_name ORDER BY column_name ASC; will sort in ascending order while `DESC` would do it in descending order. Sorting organizes data in such a way that helps in easy analysis and or displays.

8.Can you retrieve data from multiple tables using the SELECT statement?

You can get values from different tables using the SELECT statement with various types of joins like INNER JOIN, LEFT JOIN and RIGHT JOIN. These joins enable fetching data from more than two correlated tables based on a common column. For example, SELECT column_name FROM table1 INNER JOIN table2 ON table1.column = table2.column;. It fetches the values from both the tables by comparing the column values.

9.How do you use a SELECT statement to include the contents of two tables?

You use the JOIN clause to specify how the two tables are related. The most common type is INNER JOIN that returns only rows with matching values in both tables. For example, SELECT table1.column, table2.column FROM table1 INNER JOIN table2 ON table1.common_column = table2.common_column; it will bring data from both the tables where the value in common_columns matches. Other joins like LEFT JOIN or RIGHT JOIN can be used accordingly based on the output.

10.What is CROSS JOIN?

A CROSS JOIN is a kind of join which returns the Cartesian product of two tables. This means that it combines each row of the first table with every row of the second table regardless of whether there is a match between the rows. For example, SELECT * FROM table1 CROSS JOIN table2; will return a result set containing every possible combination of rows between the two tables. This join type is helpful in cases where you need all possible pairings, for example, when creating combinations or permutations.

11.How do you apply the SELECT statement to filter data?

The WHERE clause is essentially to put restrictions limiting the rows that can qualify to the results. For example, SELECT column_name FROM table_name WHERE condition says that it will return only those rows for which the condition is true.
The conditions can be of various kinds such as =, >, < or LIKE pattern matching, and this is how we filter data within those predetermined parameters, ranges or text referents.

12.How can you handle duplicate rows in SQL statements?

The DISTINCT keyword found in SQL is capable of removing duplicate rows from appearing in queries.
SELECT DISTINCT column_name FROM table_name;
returns unique values in a specified column, thereby removing any repeated rows.
To eliminate columns expected with duplicates that are undesired, it works very well. For example anomalies could be in producing a listings of unique products IDs or records customer IDs.

13.What is GROUP BY used for?

The GROUP BY clause in SQL groups rows that have the same values in specified columns into summary rows. It is most often used in conjunction with aggregate functions, such as COUNT(), SUM(), AVG(), MAX(), and MIN(), to create calculations across all the rows in one group. Example: SELECT column_name, COUNT(*) FROM table_name GROUP BY column_name; you will group the data on the column you specify and return the count of rows for each of those groups. This is useful in scenarios where you want to summarize data, like summing sales per product.

14.What is the purpose of using the HAVING clause in SQL?

The HAVING clause in SQL is used when the situation is such that you need to filter results after a GROUP BY operation has taken place. The HAVING clause filters groups of rows resulting from the GROUP BY clause, while WHERE filters rows before doing the grouping. For example, the following statement would return groups where the count of rows is more than 5: SELECT column_name, COUNT(*) FROM table_name GROUP BY column_name HAVING COUNT(*) > 5; This allows for filtering on grouped data that can't be done in WHERE - exclude groups that do not meet some criteria.

15. Can you select multiple columns in one go from multiple tables?

Yes. You can select multiple columns in Inner Join or Outter Join. Suppose the tables are related. This can be achieved using Join statement for columns from both tables.
For example,
SELECT table1.column_name, table2.column_name FROM table1 INNER JOIN table2 ON table1.id = table2.id;
gives commands to retrieve two different tables based on the match in the "id" column. This way, you can concatenate across several columns so that you can easily get complete information from several parts of the database.

16.How do you select records based on a pattern?

You have to use the LIKE clause for matching the pattern-based records. This LIKE operator is a great savior if you have to match text strings, with wildcards generally used to establish the required pattern.
SELECT * FROM table_name WHERE column_name LIKE ‘A%’;
it returns all the records which have column value beginning with the letter 'A'. The ‘%’ symbol denotes for any character or characters, whereas ‘-’ stands for exactly one. This feature allows searching or filtering of data with a desired pattern.

17.How do I use the IN operator in a SELECT statement?

IN operator uses a filter based on a list of values meaning you can check a column's value to be one of a given set.
Eg-
SELECT * FROM table_name WHERE column_name IN('value1', 'value2', 'value3');
This returns rows where the column matches one of the listed values. Writing it this way can be even more efficient than writing multiple instances of OR, and typically it is used to compare a column against a predefined list of values.

18.How do you use BETWEEN in a SELECT query?

The BETWEEN operator is applied in a SELECT query to filter data based on a range of values. It selects those records where the column value will be between the specified range, inclusive of the boundary values. For example, SELECT * FROM table_name WHERE column_name BETWEEN 10 AND 20; This statement will return all rows whose ‘column_name’ has a value between 10 and 20, inclusive 10 and 20. BETWEEN can be used for numbers or dates or text so it is very useful in retrieving records in some range such as dates within one month or prices within some range.

19.Explain how you fetch data from the table which contains NULL values?

You would use the IS NULL condition in the WHERE clause to fetch data from a table that has NULL values.
For example,
SELECT * FROM table_name WHERE column_name IS NULL;
returns rows that contain NULL values in the column mentioned. On the contrary, if you want to include rows where a column doesn't have NULL values, then you'd use IS NOT NULL. This is particularly handy when dealing with incomplete or missing data in a database as NULL values signify the lack of data in many columns.

20.How do you use SELECT INTO?

SELECT INTO is the command in SQL through which we create a new table and insert data into it from the results of a query.
For instance,
SELECT * INTO new_table FROM old_table;
This would create a new table called new_table, then copy all the data from the old_table into it. This is useful if you want to build a new table as a copy to save some portion of the data, or to do some transformation of the data before building a new table.

21.What is a use for conditions in the selection of a subset of data from a table?

The use of conditions for selecting a subset of data from a table.
The WHERE clause is used against conditions to select a subset of data from the table.
For example:
SELECT * FROM table_name WHERE column_name = 'value';
This will only retrieve those rows where the value of column_name equals 'value'. Among the other comparison operators are '=', '>', '<', 'LIKE', 'IN', 'BETWEEN', and many more that let you filter according to a certain criterion such as falling within a specific range of dates or matching against a certain exact pattern.

22. How do you use the DISTINCT keyword so that you receive no duplicate records?

DISTINCT keyword eliminates copies of rows from the resultant set of a SELECT statement.
For instance, this SELECT statement will return only unique values in that specific column without repeating them.
SELECT DISTINCT column_name FROM table_name.
When you want to fetch unique records, such as unique customer names or product categories, without repeating any value, this can come quite handy.