The documentation states,
In standard SQL, the syntax DECIMAL(M) is equivalent to DECIMAL(M,0). Similarly, the syntax DECIMAL is equivalent to DECIMAL(M,0), where the implementation is permitted to decide the value of M. MySQL supports both of these variant forms of DECIMAL syntax.
The part regarding DECIMAL(M) or DECIMAL(M,0) that I find confusing is,
For DECIMAL(M,0), the implementation is permitted to decide the value of M.
If a column is set to DECIMAL(65), 4.321 would be stored as 4, so what exactly does the documentation mean?
By running this test in MySQL 5.6:
create table t(
deci decimal,
deci4 decimal(4),
deci40 decimal(4,0),
deci42 decimal(4,2)
);
insert into t(deci, deci4, deci40, deci42)
values
(1234.1234,1234.1234,1234.1234,1234.1234);
The insert fails because 1234.1234 doesn't fit in (4,2)
insert into t(deci, deci4, deci40, deci42)
values
(1234.1234,1234.1234,1234.1234,34.1234);
Runs with the following values inserted:
deci deci4 deci40 deci42
1234 1234 1234 34.12
In DECIMAL(M,N), M is the number of places the number can take, including the decimals, N is the number of decimal places.
Maximum value for M is 65, and maximum value for N is 30.
N has to be smaller than M.
If N=0 it is 0 decimal places.
If N is not defined, the decimal places seem to be defined by the number of places actually taken by the integer part of the number, which is less than M, and the the rest of the places left for the decimals as needed.
If neither are defined then M is defaulted to 10.
Info on default values taken from here: http://www.mysqltutorial.org/mysql-decimal/