Select

SELECT Queries (SQL Server)

SQL Server SELECT queries are used to retrieve data from one or more tables in a SQL Server database. This data can be returned in a variety of ways, such as retrieving specific columns, limiting the number of rows returned, and sorting the data. In this article, we will take a detailed look at SQL Server SELECT queries and the different options and clauses that can be used to customize the data retrieval.

Retrieving Specific Columns with SELECT

To retrieve specific columns from a table, you can use the SELECT statement followed by the name of the columns you want to retrieve. For example, if you have a table called employees with columns id, name, age, and salary, you could retrieve only the name and salary columns as follows:

SELECT name, salary
FROM employees;

This SELECT statement retrieves the name and salary columns from the employees table. If you want to return all columns from employeesyou can use *instead.

SELECT *
FROM employees;

Limiting the Number of Rows with TOP

By default, a SELECT statement will retrieve all rows from a table. However, you can limit the number of rows returned by using the TOP clause. The TOP clause allows you to specify the number of rows you want to retrieve.

Example:

SELECT TOP 10 name, salary
FROM employees;

This example retrieves the first 10 rows from the employees table, and only the name and salary columns are returned.

Filtering Data with WHERE

To filter the data returned by a SELECT statement, you can use the WHERE clause. The WHERE clause allows you to specify conditions that must be met in order for a row to be returned.

Example:

SELECT name, salary
FROM employees
WHERE salary > 50000;

This example retrieves the name and salary columns from the employees table, but only for rows where the salary is greater than $50,000.

Sorting Data with ORDER BY

You can sort the data returned by a SELECT statement by using the ORDER BY clause. The ORDER BY clause allows you to sort the data based on one or more columns. By default, the data is sorted in ascending order, but you can sort it in descending order by using the DESC keyword.

Example:

SELECT name, salary
FROM employees
ORDER BY salary DESC;

This example retrieves the name and salary columns from the employees table, and the data is sorted in descending order by the salary column.

Grouping Data with GROUP BY

You can group data by using the GROUP BY clause. The GROUP BY clause allows you to group the data based on one or more columns. This can be useful for aggregating data, such as counting the number of employees in each department.

Example:

SELECT department, COUNT(*)
FROM employees
GROUP BY department;

This example retrieves the department column and the count of employees in each department, grouped by the department column. The COUNT(*) function is used to count the number of employees in each group.

Joining Tables with JOIN

You can join multiple tables by using the JOIN clause.

The JOIN clause allows you to combine data from two or more tables based on a related column between them.

There are several types of joins in SQL Server, including:

• INNER JOIN: This type of join only returns rows that have matching values in both tables.
• LEFT JOIN (or LEFT OUTER JOIN): This type of join returns all the rows from the left table, and the matching rows from the right table. If there is no matching value in the right table, NULL values are returned.
• RIGHT JOIN (or RIGHT OUTER JOIN): This type of join returns all the rows from the right table, and the matching rows from the left table. If there is no matching value in the left table, NULL values are returned.
• FULL JOIN (or FULL OUTER JOIN): This type of join returns all the rows from both tables, and the matching values from both tables. If there is no matching value in one of the tables, NULL values are returned.

Example:

SELECT employees.name, departments.department
FROM employees
JOIN departments ON employees.department_id = departments.id;

This example retrieves the name column from the employees table, and the department column from the departments table. The JOIN clause is used to combine the data from the two tables based on the department_id column in the employees table and the id column in the departments table.

Conclusion

SQL Server SELECT queries are a powerful tool for retrieving data from a database.

This should have now provided a starting point with the different options and clauses that can be used to customize the data retrieval, including retrieving specific columns, limiting the number of rows returned, filtering data, sorting data, grouping data, and joining tables.

As a starting point you should be able to start to write more complicated SELECT statements to retrieve the data you need from your SQL Server database.