Counting Rows
The COUNT()
function in SQL is used to return the number of rows that match a specified condition.
Use Case #1: Count the number of rows in a table
SELECT COUNT(*) FROM table_name;
We are running the COUNT()
function on all our attributes from a given table. This will count the number of rows.
We can also just target specific column by providing a column_name
instead of *
SELECT COUNT(col_name) FROM table_name;
Use Case #2: Counting the number of records that match a condition
A more useful use of counting would be to see how many rows will match a certain condition
SELECT COUNT(*) FROM table_name WHERE condition;
By using the WHERE
clause with COUNT
, we can select more meaningful data when required.
Use Case #3: Counting the distinct values that populates an attribute
In a scenario where an attribute can have repeating values per records (example: product type sold), it is beneficial to count the number distinct values within that attribute
SELECT COUNT(DISTINCT column_name) FROM table_name;
In the SQL query above, we are using the DISTINCT
keyword to not counting repeating values
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