DB2 ‘Aggregate’ function tricky questions

  • An aggregate function receives a set of values for each argument (such as the values of a column) and returns a single-value result for the set of input values.
  •  The following information applies to all aggregate functions, except for the COUNT(*) and COUNT_BIG(*), variations of the COUNT and COUNT_BIG functions, and the XMLAGG function.
  •  The argument of an aggregate function is a set of values derived from an expression. The expression must not include another aggregate function or a scalar fullselect. The scope of the set is a group or an intermediate result table, as explained in the information on the GROUP BY clause.
  •  If a GROUP BY clause is specified in a query and the intermediate result from the FROM, WHERE, GROUP BY, and HAVING clauses is the empty set, then the aggregate functions are not applied and the result of the query is the empty set.
  •  If the GROUP BY clause is not specified in a query and the intermediate result table of the FROM, WHERE, and HAVING clauses is the empty set, then the aggregate functions are applied to the empty set.
  •  For example, the result of the following SELECT statement is the number of distinct values of JOB for employees in department D11:SELECT COUNT(DISTINCT JOB)
    FROM DSN8A10.EMP
    WHERE WORKDEPT = ‘D11’;
    The keyword DISTINCT is not an argument of the function but rather a specification of an operation that is performed before the function is applied. If DISTINCT is specified, redundant duplicate values are eliminated. If ALL is implicitly or explicitly specified, redundant duplicate values are not eliminated.DISTINCT must not be specified preceding an XML value.
  •  For compatibility with other SQL implementations, UNIQUE can be specified as a synonym for DISTINCT in aggregate functions.
  • When interpreting the DISTINCT clause for decimal floating-point values that are numerically equal, the number of significant digits in the value is not considered. For example, the decimal floating-point number 123.00 is not distinct from the decimal floating-point number 123. The representation of the number returned from the query will be any one of the representations encountered.
  •  An aggregate function can be used in a WHERE clause only if that clause is part of a subquery of a HAVING clause and the column name specified in the expression is a correlated reference to a group. If the expression includes more than one column name, each column name must be a correlated reference to the same group.
  •  The result of the COUNT and COUNT_BIG functions cannot be the null value. As specified in the description of AVG, MAX, MIN, STDDEV, SUM, and VARIANCE, the result is the null value when the function is applied to an empty set. However, the result is also the null value when the function is specified in an outer select list, the argument is given by an arithmetic expression, and any evaluation of the expression causes an arithmetic exception (such as division by zero).
  •  If the argument values of an aggregate function are strings from a column with a field procedure, the function is applied to the encoded form of the values and the result of the function inherits the field procedure.

Ref:IBM

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Author: Srini

Experienced software developer. Skills in Development, Coding, Testing and Debugging. Good Data analytic skills (Data Warehousing and BI). Also skills in Mainframe.