Basic Queries ​

To query data from a database table, we use the following SQL statement:

SELECT * FROM <table_name>

Assuming the table name is students, to query all rows from the students table, we use the following SQL statement:

-- Query all data from the students table
SELECT * FROM students;

When using SELECT * FROM students, SELECT is a keyword indicating a query, * represents "all columns," and FROM indicates which table to query, in this case, the students table. This SQL will retrieve all data from the students table, and the result will also be a two-dimensional table containing column names and each row of data.

To query all rows from the classes table, we use:

-- Query all data from the classes table
SELECT * FROM classes;

Running the above SQL statement will show the query results.

The SELECT statement does not always require a FROM clause. For example:

-- Calculate 100 + 200
SELECT 100 + 200;

This query will directly compute the result of the expression. Although SELECT can be used for calculations, it is not its primary strength. However, a SELECT statement without a FROM clause is useful for checking if the current database connection is valid. Many monitoring tools execute SELECT 1; to test the database connection.

Summary ​

The basic SELECT query SELECT * FROM <table_name> can retrieve all rows and all columns of data from a table. The result of a SELECT query is a two-dimensional table.