ODBC and JDBC Developer’s Guide

Chapter 4
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Supported standards 33
WHERE clause
The WHERE clause specifies the conditions that records must meet to be retrieved. The WHERE clause
contains conditions in the form:
WHERE expr1 rel_operator expr2
expr1 and expr2 can be field names, constant values, or expressions.
rel_operator is the relational operator that links the two expressions. For example, the following
SELECT statement retrieves the names of employees who make $20,000 or more.
SELECT last_name,first_name FROM emp WHERE salary >= 20000
Note If you use fully qualified names in the SELECT (projection) list, you must also use fully qualified
names in the related WHERE clause.
GROUP BY clause
The GROUP BY clause specifies the names of one or more fields by which the returned values should be
grouped. This clause is used to return a set of aggregate values. It has the following format:
GROUP BY column_expressions
column_expressions must match the column expression used in the SELECT clause. A column
expression can be one or more field names of the database table separated by commas, or one or more
expressions separated by commas.
The following example sums the salaries in each department.
SELECT dept_id, sum(salary) FROM emp GROUP BY dept_id
This statement returns one row for each distinct department ID. Each row contains the department ID and
the sum of the salaries of the employees in the department.
HAVING clause
The HAVING clause enables you to specify conditions for groups of records (for example, display only the
departments that have salaries totaling more than $200,000). This clause is valid only if you have already
defined a GROUP BY clause. It has the following format:
HAVING expr1 rel_operator expr2
expr1 and expr2 can be field names, constant values, or expressions. These expressions do not have to
match a column expression in the SELECT clause.
rel_operator is the relational operator that links the two expressions. The following example returns
only the departments whose sums of salaries are greater than $200,000:
SELECT dept_id, sum(salary) FROM emp
GROUP BY dept_id HAVING sum(salary) > 200000
UNION operator
The UNION operator combines the results of two or more SELECT statements into a single result. The
single result is all of the returned records from the SELECT statements. By default, duplicate records are
not returned. To return duplicate records, use the ALL keyword (UNION ALL). The format is:
SELECT statement UNION [ALL] SELECT statement