How to prevent a SELECT from reading after another transaction is done?

[Ariel Arelovich]

Here is the thing. I need to insert a row into a table but ONLY if the number of rows is less than 10.

Normally I do this by:
1) SELECT COUNT(*) FROM mytable WHERE cDate = '2019-01-01';
2) Check that the return value is less than 10. If not, then I need to return a message to the user.
3) If it is less than 10. I do INSERT INTO mytable (cDate) VALUE ('2019-01-01').


Now this is running in a server and the table has 9 rows. So if while I'm doing the check on the number of rows, ANOTHER user does the same check it will again return 9 rows (because the INSERT has not happened yet). This will result in two inserts giving a total of 11 rows where the maximum allowed is 10.

How can I prevent this from happening?