Functions on Null Values

Binary functions or operators involving null values return null results.

x = 1.0 + 5.6 * 3 + NULL + 3
isNull(x)
// output: 1
typestr x;
// output: DOUBLE

5 + NULL      // return null

x=1 2 NULL NULL 3;
y=2 NULL NULL 3 4;
x+y;
// output: [3,,,,7]

false || 00i     // return null
true && 00i      // return null

In comparison operators >, >=, <, <=, and between, null values are treated as the minimum value by default. To have comparisons involving null values return a logical null result, set the configuration parameter nullAsMinValueForComparison to false. Nonetheless, in operators !=, <>, and ==, null values are always treated as the minimum regardless of this parameter. Therefore, in DolphinDB, comparing two null values for equality returns true, and for inequality returns false.

// nullAsMinValueForComparison = true
1 < NULL // output: false
1 > NULL // output: true
NULL == NULL // output: true
NULL != NULL // output: false

// nullAsMinValueForComparison = false
1 < NULL // output: 00b
1 > NULL // output: 00b
NULL == NULL // output: true
NULL != NULL // output: false

In logical operators and and xor, null values are treated the same as in basic arithmetic operations: if any operand is null, the result is null.

false || 00i     // return null
true && 00i      // return null

The logical operator or processes null values differently. By default (with logicOrIgnoreNull=true), if only one operand is null, the result matches the other non-null operand. If both operands are null, the result is also null. However, when logicOrIgnoreNull is set to false, any null operand leads to a null result.

// logicOrIgnoreNull = true
NULL or true // output: true
NULL or false // output: false
NULL or NULL // output: 00b

// logicOrIgnoreNull = false

NULL or true // output: 00b
NULL or false // output: 00b
NULL or NULL // output: 00b

Aggregate functions such as sum/avg/med ignore null values, as do vector functions involving these aggregate functions, such as msum and cumsum.

x = 1 2 NULL NULL 3;
log x;
// output: [0,0.693147,,,1.098612]

avg x;
// output: 2

sum x;
// output: 6

med x;
// output: 2

cumsum(x)
// output: 1 3 3 3 6

In functions ols/olsEx/corrMatrix/olsEx, null values in parameters are replaced with 0.

m = take(rand(10.0, 20) join NULL, 30) $10:3
corrMatrix(m)