In Hierarchical queries using the Oracle RDBMS - Determine ordering within each level, I showed how to sort the rows of a hierarchical SQL query within each tree level by using ORDER SIBLINGS BY instead of ORDER BY.
There is one detail to keep in mind, though: even the ORDER SIBLINGS BY is done on the overall result of the query, which can cause problems when using UNION ALL to take additional rows into the result:
SELECT 0, 0, 'xyz' FROM DUAL UNION ALL SELECT LEVEL, Id, substr( LPAD (' ', 3 * (LEVEL - 1)) || Label, 0, 30) Node FROM TreeNode START WITH Parent = 0 CONNECT BY PRIOR Id = Parent ORDER SIBLINGS BY Label;
ERROR at line 7: ORA-30929: ORDER SIBLINGS BY clause not allowed here
Using parantheses to explicitly group the second select within the UNION ALL does not help, but there is a simple solution: just wrap the hierarchical query into a sub select, and then do a SELECT * from the sub select:
SELECT 0, 0, 'xyz' FROM DUAL UNION ALL SELECT * FROM ( SELECT LEVEL, Id, substr( LPAD (' ', 3 * (LEVEL - 1)) || Label, 0, 30) Node FROM TreeNode START WITH Parent = 0 CONNECT BY PRIOR Id = Parent ORDER SIBLINGS BY Label );
0 0 'XYZ' ------------------ ------------------ --------- 0 0 xyz 1 1 Root Node 2 2 Node 1 3 3 Node 1.1 3 4 Node 1.2 ...
This article shows how to execute hierarchical queries using the Oracle database: Hierarchical queries using the Oracle RDBMS
GROUP BY example
The GROUP BY clause is used to group selected rows by a certain expression, means it groups all rows for which the given expression returns the same value. Then, for each group, a single result is calculated by using aggregate functions (the number of result rows is the same as the number of groups). See Oracle® Database SQL Language Reference: group_by_clause for more information.
As an example, lets assume we have some revenue information for each month:
Description Month Revenue ----------- --------- ------- Deal 1 January 100 Deal 2 January 150 Deal 3 February 200 Deal 4 February 50 Deal 5 March 120
Using GROUP BY, we can group the data by Month and calculate a single result for each group, for example the summary of all revenues for each month:
Description Month Revenue ----------- --------- ------- Deal 1 January 100 Deal 2 January 150 =========================== Summary: 250 Deal 3 February 200 Deal 4 February 50 =========================== Summary: 250 Deal 5 March 120 =========================== Summary: 120
The result will then only contain the three rows with the summarized revenue values.
GROUP BY SQL query
Lets see how this works in SQL - first, we create a table with some data, according to the example from above:
CREATE TABLE t_revenue ( c_id NUMBER(18) NOT NULL, c_description VARCHAR2(100) NOT NULL, c_month VARCHAR2(20) NOT NULL, c_amount NUMBER(18) NOT NULL ); INSERT INTO t_revenue VALUES(0, 'Deal 1', 'January', 100); INSERT INTO t_revenue VALUES(1, 'Deal 2', 'January', 150); INSERT INTO t_revenue VALUES(2, 'Deal 3', 'February', 200); INSERT INTO t_revenue VALUES(3, 'Deal 4', 'February', 50); INSERT INTO t_revenue VALUES(4, 'Deal 5', 'March', 120); COMMIT;
The most simple example calculates the summary of all revenues for each month, as above:
SELECT SUM(c_amount) -- The expression (aggregate function) which is applied to each group of rows FROM t_revenue GROUP BY c_month; -- The expression by which to group the rows
SUM(C_AMOUNT) ------------- 120 250 250
While this query works as expected, we can not see which month the results refer to - but we can simply add the month column to the result:
SELECT c_month, SUM(c_amount) FROM t_revenue GROUP BY c_month;
C_MONTH SUM(C_AMOUNT) -------------------- ------------- March 120 January 250 February 250
(Note that, with this data model, it is not easy to sort by the month - we can not simply ORDER BY c_month because that would cause "February" to appear before "January").
ORA-00979: not a GROUP BY expression
One error which often occurs with GROUP BY is ORA-00979: not a GROUP BY expression. That means what we have added an expression to the projection of the SQL statement (the columns which we want to select) which would not result in a single value. For example, we could try to add the c_description column as follows:
SELECT c_description, c_month, SUM(c_amount) FROM t_revenue GROUP BY c_month;
But since the c_description column can not be condensed to a single value for each group (the values neither have the same value within the group as specified in the GROUP BY clause, nor are they condensed to a single value through an aggregate function) we get:
SELECT c_description, c_month, SUM(c_amount) * ERROR at line 1: ORA-00979: not a GROUP BY expression
However, we could use another aggregate function, for example LISTAGG which returns a single result by concatenating the values of a given column for each row within the group. The LISTAGG function also requires some additional syntax to define the ordering of the values within the list. See the LISTAGG documentation for more information.
SELECT LISTAGG(c_description, ';') WITHIN GROUP (ORDER BY c_description) Deals, c_month, SUM(c_amount) FROM t_revenue GROUP BY c_month;
DEALS C_MONTH SUM(C_AMOUNT) ---------------- ------------ ------------- Deal 3;Deal 4 February 250 Deal 1;Deal 2 January 250 Deal 5 March 120
There is sometimes a little confusion about the term Schema in Oracle databases. In General, a database schema is a collection of database objects (Tables, Views, Packages, ...) which conceptually belong together.
However, in Oracle RDBMS, a schema is also tightly coupled to a user. Essentially, once there is a user, there is a schema (which can be empty though, as for user "RUN" in the following diagram):
The Schema name is the same as the user name - generally, in Oracle databases, the terms "User" and "Schema" are the same.
A typical deployment scenario is to have a runtime user ("RUN" in the example above) who does not own any database objects, and an owning user who owns the database objects. The application then accesses the database through the runtime user, which can be granted or denied access to the various database objects which are owned by the owning user.
See also Difference between a User and a Schema in Oracle? on stackoverflow for more information.