EQL is, in many ways, similar to SQL, but has some marked
differences as well.
This
topic identifies EQL concepts that may be familiar to users familiar with SQL,
as well as the unique features of EQL:
- Tables with a single
schema vs a corpus of records with more than one schema. SQL is designed
around tables of records — all records in a table have the same schema. EQL is
designed around a single corpus of records with heterogeneous schemas.
- EQL Query vs SQL
Query. An EQL statement requires a
DEFINE or
RETURN clause, which, like a SQL common table
expression (or CTE), defines a temporary result set. The following differences
apply, however:
- EQL does not support a
schema declaration.
- In EQL, the scope of a
CTE is the entire query, not just the immediately following statement.
- In EQL, a
RETURN is both a CTE and a normal statement (one
that produces results).
- EQL does not support
recursion. That is, a statement cannot refer to itself using a
FROM clause, either directly or indirectly.
- EQL does not contain an
update operation.
- Clauses. In EQL,
SELECT,
FROM,
WHERE,
HAVING,
GROUP BY, and
ORDER BY are all like SQL, with the following caveats:
- In
SELECT statements,
AS aliasing is optional when selecting an
attribute verbatim; statements using expressions require an
AS alias. Aliasing is optional in SQL.
- In EQL,
GROUP BY implies
SELECT. That is, grouping attributes are always
included in statement results, whether or not they are explicitly selected.
- Grouping by a
multi-assign attribute can cause a single record to participate in multiple
groups.
- WHERE
can be applied to an aggregation expression.
- In SQL, use of
aggregation implies grouping. In EQL, grouping is always explicit.
- Other language
differences.
- PAGE
works in the same way as many common vendor extensions to SQL.
- In EQL, a
JOIN expression's Boolean join condition must be
contained within parentheses. This is not necessary in SQL.
- EQL supports
SELECT statements only. It does not support other
DML statements, such as
INSERT or
DELETE, nor does it support DDL, DCL, or TCL
statements.
- EQL supports a different
set of data types, expressions, and functions than described by the SQL
standard.
