CSV LIST

Group Aggregation

-- Below is the function to calculate the group aggregation

CREATE OR REPLACE
FUNCTION to_string(
str_in IN agg_string,
delimiter_in IN VARCHAR2 DEFAULT ',',
skip_empty IN VARCHAR2 DEFAULT 'Y')
RETURN VARCHAR2
IS
v_index INTEGER;
v_str_data VARCHAR2(32767) ;
v_delimiter VARCHAR2(31) ;
BEGIN
v_index:=str_in.FIRST;
WHILE v_index IS NOT NULL
LOOP
IF NOT(skip_empty='Y' AND str_in(v_index) IS NULL) THEN
v_str_data:=v_str_data || v_delimiter || str_in(v_index) ;
v_delimiter:=delimiter_in;
END IF;
v_index:=str_in.NEXT(v_index) ;
END LOOP;
RETURN v_str_data;
END to_string;


-- The type variable to hold the csv list

CREATE OR REPLACE TYPE agg_string
AS
TABLE OF VARCHAR2(4000) ;

-- The sql that can be used to generate the list is as follows

SELECT dept, to_string(CAST(collect(employee_id) AS agg_string),',') concatenated FROM employee GROUP BY dept;

Join (SQL)

A SQL JOIN clause combines records from two or more tables in a database.^[1] It creates a set that can be saved as a table or used as is. A JOIN is a means for combining fields from two tables by using values common to each. ANSI standard SQL specifies four types of JOINs: INNER, OUTER, LEFT, and RIGHT. In special cases, a table (base table, view, or joined table) can JOIN to itself in a self-join.

Inner join

An inner join is the most common join operation used in applications and can be regarded as the default join-type. Inner join creates a new result table by combining column values of two tables (A and B) based upon the join-predicate. The query compares each row of A with each row of B to find all pairs of rows which satisfy the join-predicate. When the join-predicate is satisfied, column values for each matched pair of rows of A and B are combined into a result row. The result of the join can be defined as the outcome of first taking the Cartesian product (or cross-join) of all records in the tables (combining every record in table A with every record in table B)—then return all records which satisfy the join predicate. Actual SQL implementations normally use other approaches like a Hash join or a Sort-merge join where possible, since computing the Cartesian product is very inefficient.

SQL specifies two different syntactical ways to express joins: "explicit join notation" and "implicit join notation".

The "explicit join notation" uses the JOIN keyword to specify the table to join, and the ON keyword to specify the predicates for the join, as in the following example:

SELECT *

FROM employee INNER JOIN department

 ON employee.DepartmentID = department.DepartmentID;

The "implicit join notation" simply lists the tables for joining (in the FROM clause of the SELECT statement), using commas to separate them. Thus, it specifies a cross-join, and the WHERE clause may apply additional filter-predicates (which function comparably to the join-predicates in the explicit notation).

The following example shows a query which is equivalent to the one from the previous example, but this time written using the implicit join notation:

SELECT *

FROM   employee, department

WHERE  employee.DepartmentID = department.DepartmentID;

The queries given in the examples above will join the Employee and Department tables using the DepartmentID column of both tables. Where the DepartmentID of these tables match (i.e. the join-predicate is satisfied), the query will combine the LastName, DepartmentID and DepartmentName columns from the two tables into a result row. Where the DepartmentID does not match, no result row is generated.

Thus the result of the execution of either of the two queries above will be:

Employee.LastName	Employee.DepartmentID	Department.DepartmentName	Department.DepartmentID
Robinson	34	Clerical	34
Jones	33	Engineering	33
Smith	34	Clerical	34
Steinberg	33	Engineering	33
Rafferty	31	Sales	31

Note: Programmers should take special care when joining tables on columns that can contain NULL values, since NULL will never match any other value (or even NULL itself), unless the join condition explicitly uses the IS NULL or IS NOT NULL predicates.

Notice that the employee "John" and the department "Marketing" do not appear in the query execution results. Neither of these has any matching records in the respective other table: "John" has no associated department, and no employee has the department ID 35. Thus, no information on John or on Marketing appears in the joined table. Depending on the desired results, this behavior may be a subtle bug. Outer joins may be used to avoid it.

One can further classify inner joins as equi-joins, as natural joins, or as cross-joins.

Equi-join

An equi-join, also known as an equijoin, is a specific type of comparator-based join, or theta join, that uses only equality comparisons in the join-predicate. Using other comparison operators (such as <) disqualifies a join as an equi-join. The query shown above has already provided an example of an equi-join:

SELECT *

FROM   employee

     EQUI JOIN department

        ON employee.DepartmentID = department.DepartmentID;

SQL provides an optional shorthand notation for expressing equi-joins, by way of the USING construct (Feature ID F402):

SELECT *

FROM   employee

     INNER JOIN department

        USING (DepartmentID);

The USING construct is more than mere syntactic sugar, however, since the result set differs from the result set of the version with the explicit predicate. Specifically, any columns mentioned in the USING list will appear only once, with an unqualified name, rather than once for each table in the join. In the above case, there will be a single DepartmentID column and no employee.DepartmentID or department.DepartmentID.

The USING clause is not supported by SQL Server 2005.

Natural join

A natural join offers a further specialization of equi-joins. The join predicate arises implicitly by comparing all columns in both tables that have the same column-name in the joined tables. The resulting joined table contains only one column for each pair of equally-named columns.

The above sample query for inner joins can be expressed as a natural join in the following way:

SELECT *

FROM   employee NATURAL JOIN department;

As with the explicit USING clause, only one DepartmentID column occurs in the joined table, with no qualifier:

DepartmentID	Employee.LastName	Department.DepartmentName
34	Smith	Clerical
33	Jones	Engineering
34	Robinson	Clerical
33	Steinberg	Engineering
31	Rafferty	Sales

Cross join

Example of an explicit cross join:

SELECT *

FROM   employee CROSS JOIN department;

Example of an implicit cross join:

SELECT *

FROM   employee, department;

Employee.LastName	Employee.DepartmentID	Department.DepartmentName	Department.DepartmentID
Rafferty	31	Sales	31
Jones	33	Sales	31
Steinberg	33	Sales	31
Smith	34	Sales	31
Robinson	34	Sales	31
John	NULL	Sales	31
Rafferty	31	Engineering	33
Jones	33	Engineering	33
Steinberg	33	Engineering	33
Smith	34	Engineering	33
Robinson	34	Engineering	33
John	NULL	Engineering	33
Rafferty	31	Clerical	34
Jones	33	Clerical	34
Steinberg	33	Clerical	34
Smith	34	Clerical	34
Robinson	34	Clerical	34
John	NULL	Clerical	34
Rafferty	31	Marketing	35
Jones	33	Marketing	35
Steinberg	33	Marketing	35
Smith	34	Marketing	35
Robinson	34	Marketing	35
John	NULL	Marketing	35

The cross join does not apply any predicate to filter records from the joined table. Programmers can further filter the results of a cross join by using a WHERE clause.