w3resource: "SQL Tutorial"

Review the following pages in this tutorial: SELECT with DISTINCT (under SQL SELECT STATEMENT), WHERE CLAUSE, and SQL AGGREGATE FUNCTIONS. 

DISTINCT is a clause of the Select statement, which specifies that the data retrieved from the database be distinct. Suppose a student database contained data on student computer science courses, and the desired data are the names of all the students taking a computer science course. If a student was taking 2 computer science courses, there would be 2 course records in the database, each having that students name. The DISTINCT clause specifies that the student name be returned just once, not twice.

The SELECT statement can be used to find specific data in specific records. It can also be used to search or browse a database. The WHERE clause is used to specify conditions on the data that is queried. Fewer conditions result in more data found; more conditions result in less data being found.

Logical operators, such as AND, NOT, are used to combine conditions to place more restrictions on the data desired, and, thus, filter out more undesired data. The SQL DML language manipulates data from a database. It is used to sort data, arrange data, or perform calculations on the data. Aggregate functions are built in DML functions used in the Select statement to summarize an amount of data, and return a calculated or summary value, instead of returning all of the data found. For example, COUNT, AVERAGE, SUM, MAX, and MIN are aggregate functions. Note that a function takes arguments, written in parentheses after the function name.