You can also use the WHERE clause along with the GROUP BY clause. The GROUP BY clause will go after the WHERE clause like so:

USE lahmansbaseballdb;
SELECT playerid, teamid 
FROM batting
WHERE playerid = 'aardsda01'
GROUP BY teamid, playerid;

The previous query returns the results shown in the following screenshot: 

Here, you can see that you are now only getting the results for aardsda01. 

As a comparison, if you execute the previous query without GROUP BY, the query will return one additional row: 

USE lahmansbaseballdb;
SELECT playerid, teamid
FROM batting
WHERE playerid = 'aardsda01';

The following screenshot shows the results of running the previous query: 

You can now see two rows containing style. This is because this player was on the Seattle team for 2 years, whereas he was only on the other teams for 1 year. You can view this information by executing the following query: 

USE lahmansbaseballdb
SELECT playerid, teamid, yearid
FROM batting
WHERE playerid = 'aardsda01'; 

The following screenshot shows you the results of running the previous query: 

Here, you can see that teamid is not unique because there are 2 years for the SEA team ID. This shows us that GROUP BY is like using DISTINCT.

The following table shows a comparison between GROUP BY and DISTINCT: 

Here, you can see that GROUP BY, without aggregate functions, produces the same results as DISTINCT.