Counting number of grouped rows in mysql

Question

In a table xyz I have a row called components and a labref row which has labref number as shown here

Table xyz

labref             component
NDQA201303001         


        

Answer 1

Try this simple query without a sub-query:

SELECT COUNT(DISTINCT component) AS TotalRows
FROM xyz
WHERE labref = 'NDQA201303001';

See this SQLFiddle