--------------------- Statistical Functions --------------------- By the SQL specification, some statistical functions are defined. Function about variance and standard deviation are bellow. VAR_SAMP: return the sample variance. eq. (SUM(<expr> ^ 2) - SUM(<expr>) ^ 2 / COUNT(<expr>)) / (COUNT(<expr>) - 1) VAR_POP: return the population variance. eq. (SUM(<expr> ^ 2) - SUM(<expr>) ^ 2 / COUNT(<expr>)) / COUNT(<expr>) STDDEV_SAMP: return the sample standard deviation. eq. SQRT(VAR_SAMP(<expr)) STDDEV_POP: return the population standard deviation. eq. SQRT(VAR_POP(<expr>)) COVAR_SAMP: return the sample population. eq. (SUM(<expr1> * <expr2>) - SUM(<expr1>) * SUM(<expr2>) / COUNT(*)) / (COUNT(*) - 1) COVAR_POP: return the population covariance. eq. (SUM(<expr1> * <expr2>) - SUM(<expr1>) * SUM(<expr2>) / COUNT(*)) / COUNT(*) CORR: returns the coefficient of correlation. eq. COVAR_POP(<expr1>, <expr2>) / (STDDEV_POP(<expr2>) * STDDEV_POP(<expr1>)) Author: Hajime Nakagami <nakagami@gmail.com> Syntax: <single param statistical function> ::= <single param statistical function name>(<expr>) <single param statistical function name> := { VAR_POP | VAR_SAMP | STDDEV_POP | STDDEV_SAMP } <dual param statistical function> ::= <dual param statistical function name>(<expr1>, <expr2>) <dual param statistical function name> := { COVAR_POP | COVAR_SAMP | CORR } Note: If VAR_SAMP, STDDEV_SAMP, COVAR_SAMP and result count is 0 or 1, return NULL. If VAR_POP, STDDEV_POP, COVAR_POP, CORR and result count is 0, return NULL. Example: SELECT STDDEV_SAMP(salary) FROM employees;