There a few ways you can use the % wildcard, which represents zero or more characters. You can use it to find a value at the beginning, end, or middle of a string. For example, the following query will find all player IDs that start with the letter a: 

USE lahmansbaseballdb;
SELECT playerid, g_all, g_batting, g_defense 
FROM appearances
WHERE playerid LIKE 'a%'; 

The previous query will return the rows shown in the following screenshot: 

If you move the % sign in front of the letter a, you are filtering on anything that ends in a. In this case, it will find no rows since playerid always ends in a number:

USE lahmansbaseballdb;
SELECT playerid, g_all, g_batting, g_defense 
FROM appearances
WHERE playerid LIKE '%a'; 

The previous query will return no rows, as shown in the following screenshot:  

If you add a % sign to the end as well, then you will be filtering on any rows that have the letter a somewhere in the playerid value, as shown in the following query: 

USE lahmansbaseballdb;
SELECT playerid, g_all, g_batting, g_defense 
FROM appearances
WHERE playerid LIKE '%a%';

The results of the previous query will provide the rows shown in the following screenshot: 

You can also place multiple characters before, after, or in-between percent wildcard. 

This query will return rows where playerid starts with the letters wr: 

USE lahmansbaseballdb;
SELECT playerid, g_all, g_batting, g_defense 
FROM appearances
WHERE playerid LIKE 'wr%';

This query will return rows where playerid contains the letters ds: 

USE lahmansbaseballdb;
SELECT playerid, g_all, g_batting, g_defense 
FROM appearances
WHERE playerid LIKE '%ds%';

To summarize, you can use the % wildcard to search for strings at the beginning, middle, or end of strings.