SQL Practice Problems: Simple Select Statements is a set of well-designed problems that will help you understand and retain your learning. It's easy to find information on SQL syntax and keywords online, but harder to come across challenging, well-designed, real-world problems. The SELECT statement is used to get data from tables. You can filter, sort and group data with the GROUP BY and DISTINCT clauses.
SELECT
The SELECT statement is used to retrieve data from a table or view. It can also be used to query multiple tables or views in a join.
The select_list identifies the data that you want to retrieve from a database. This can be anything from a column name to a calculation or function call. If the column names in a table are too long or difficult to read, sql problems for practice provides the option of using aliases. This is done by prefacing the column names with a letter or number and then adding the alias name to the end.
WHERE
SQL lets you return information based on a variety of conditions. As a coder, business rules often dictate how you structure these SQL statements to ensure precise record sets are returned for reports and applications.
The WHERE clause filters rows in the table specified by the FROM clause. It consists of one or more logical expressions called predicates that evaluate each row in the table. As a general rule, it's best to put your query aliases before the WHERE clause. This way, the aliases have been assigned by the time the WHERE clause is processed.
GROUP BY
SQL uses group by when you want to return a summary of a set of records. For example, you could query for all cities with a certain letter of the alphabet (like "Da") or by total sales.
The GROUP BY statement groups the same value rows together, so you can use aggregate functions like AVG(), COUNT(), SUM(), etc. You can also add a HAVING clause to filter the aggregate grouped data as we will see in the next lesson.
DISTINCT
The DISTINCT keyword is used in conjunction with the SELECT statement to eliminate duplicate values and fetch unique records from a table. It is often used when dealing with tables that contain multiple duplicate values, a common scenario that requires data to be fetched only once.
Unlike the GROUP BY clause, the DISTINCT clause can be applied to more than one column. However, it is recommended to use GROUP BY whenever possible as it is more flexible and reduces query performance overhead. DISTINCT also treats NULL as a unique value. For example, the following query will return all distinct course names in which students are enrolled, including NULL values.
JOIN
The JOIN keyword is used in SQL to combine rows from two tables into a single result set. There are several different types of joins, including INNER JOIN, which returns only rows that match both tables; LEFT OUTER JOIN, which reverses the relationship; and FULL OUTER JOIN, which returns all rows from both tables.
Whenever possible, it's a good idea to use the AS keyword to give new names to columns returned by your query. This is called aliasing, and it helps make the query easier to read and understand.
INSERT
The SELECT statement retrieves data from database tables in the form of result sets. It is the most popular SQL command. It can be used to retrieve existing data from one or more tables, as well as the results of various mathematical calculations. You can use the AS keyword to give new names to columns returned by your query. These are called aliases, and they make your query more understandable to people who read it. Using aliases is also a good way to reduce the load on your database.
DELETE
The DELETE statement removes data records from SQL tables. It is a very powerful and irreversible operation, so it's important to be careful with it. A DELETE query can search for records based on conditions specified in the WHERE clause. This can be useful if you need to delete rows in one table that are referenced by rows in another.
For example, you can use a DELETE query to remove employees who work in departments that have had accidents from the EMP table when there are matching records in the DEPT_ACCIDENTS table.
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