Question

1. Does bool value false == 0 and true == 1
2. Does Null value in varchar, int, date fields == 0?
3. When by default in mysql values == 0 or NULL or empty string?

Answer 1

Question: Does bool value false == 0 and true == 1

Yes – you can test using:

SELECT IF(0, 'true', 'false')

…it will return “false”, meaning zero indicates false and one indicates true. This example was taken from the MySQL documentation page.

Question: Does Null value in varchar, int, date fields == 0?

No, NULL indicates the lack of any value at all.
You need to use special operators if you want to compare for the value being NULL.

Question: When by default in mysql values == 0 or NULL or empty string?

Without default constraints in place, columns that allow NULL will contain NULL if no value has been provided — regardless of data type (DATETIME, numeric, text, etc). Columns that do not allow NULL will return an error until a value is provided that matches the data type of the column. Default constraints allow you to specify a default value if someone attempts to insert a NULL value into the column, or the column has been omitted from an INSERT statement.