You can create a CMP index on columns that are NULL, NOT NULL, or a mixture. The CMP index cannot be unique.
Numeric and decimal data types are considered identical. You may create CMP indexes on them when precision and scale are identical. For CHAR, VARCHAR, BINARY, and VARBINARY columns, precision means having the same column width.
For example, these commands create a table, then create appropriate CMP indexes:
CREATE TABLE f(c1 INT NOT NULL, c2 INT NULL, c3 CHAR(5), c4 CHAR(5))
CREATE CMP INDEX c1c2cmp ON f(c1, c2)
This index is illegal because the columns indexed are not of the same data type, precision, and scale:
CREATE CMP INDEX c1c3cmp ON f(c1, c3)