Im looking at the following two ways of retrieving a value and storing it later via an insert statement. i.e. via Pl/SQL cursors or via direct SQL.
Is there any advantage to either approach? Or is there a more efficient approach?
Approach 1
Cursor system_date
Is
select sysdate from dual;
system_date_rec system_date%type;
Open system_Date;
Fetch system_date into system_date_rec;
Insert into table(dateValue)
values(system_date_rec.date);
Approach 2
dateString varchar(20);
Select sysdate into dateString from dual;
Insert into table(dateValue)
values(dateString);
How about approach 3:
Insert into table(dateValue)
values(sysdate);
or assuming you did actually need to do a select to get the data:
Insert into table(dateValue)
select dateValue from other_table where ...;
Regarding whether an explicit cursor or a SELECT INTO is preferable when one or the other is needed, I would go for the SELECT INTO because it is neater and safer if you expect the query to return exactly one row:
select some_value
into l_var
from other_table
where ...;
if l_var = 'A' then
do_something;
end if;
Now you will get an exception (NO_DATA_FOUND or TOO_MANY_ROWS) if the number of rows returned is not as expected. With the cursor you will just end up with l_var unchanged, or set to the value from the first matching row - which probably means yu've got a bug but don't know it.
Each approach has it's merit but if it's one and only one value you are getting then I'd go with select ... into ... as this is much simpler and will check that you have one and only one value.
Although Tony's approach is possibly preferable to both in the right circumstances.
If you also want to get the value back there is always the RETURNING clause of the insert statement.
my_date_value date;
...
INSERT into table(datevalue)
values (sysdate)
returning sysdate into my_date_value;
I'd agree with #Tony and #MikeyByCrikey that select ... into is generally preferable, not least - in my personal, subjective opinion - because it keeps the select and into together instead of having the select out of sight up in the declare section. Not really an issue if it's simple but you've suggested you're doing several big queries and manipulations, which implies a longish procedure.
Slightly off-topic, but if all the manipulations are to gather data for a single insert at the end, then rather than having lots of separate variables I'd consider declaring a single variable as a row type and updating the columns as appropriate:
declare
l_row my_table%ROWTYPE;
begin
select ... into l_row.column1;
select ... into l_row.column2;
if l_row.column2 = 'A' then
/* do something */
end if;
l_row.column3 := 'somevalue';
fetch ... into l_row.column4;
/* etc */
insert into my_table values l_row;
end;
Related
Given the following example:
BEGIN
FOR r IN (
SELECT * FROM table_one WHERE change_id = 0
) LOOP
r.change_id := -1;
INSERT INTO table_one VALUES r;
END LOOP;
END;
This inserts new rows to table_one with the exact same content, except the intended change on column change_id to the value -1. I don't have to specify the columns inside of the script as I have to in an INSERT INTO table_one (change_id, ...) SELECT -1, ... FROM table_one WHERE change_id=0;
It works perfectly fine. But how to modify this script to work with a list of tables? The internal structure of those tables are different, but all of them have the necessary column change_id.
Of course the easiest solution would be to copy and paste this snippet x-times and replace the fix table name inside. But is there an option to work with a list of tables in an array?
My approach was like this:
DECLARE
TYPE tablenamearray IS VARRAY(30) OF VARCHAR2(30);
tablenames tablenamearray;
BEGIN
tablenames := tablenamearray('TABLE_ONE', 'TABLE_TWO', 'TABLE_THREE'); -- up to table 30...
FOR i IN tablenames.first..tablenames.last LOOP
/* Found no option to use tablenames(i) here with dynamic SQL */
END LOOP;
END;
Note: There is no technical primary key like an id with a sequence behind. The primary key is build by three columns incl. the change_id column.
You cannot create a SQL statement where the statement is not known at parse time. So, you cannot have a variable as a table name. What you're looking for is Dynamic SQL, which is a fairly complicated topic, but basically you're going to wind up building a SQL statement with DBMS_SQL or running a statement as a string with EXECUTE IMMEDIATE.
I'm playing around with array support in Oracle and hit a roadblock regarding array access within a SQL query. I'm using the following schema:
create type smallintarray as varray(10) of number(3,0);
create table tbl (
id number(19,0) not null,
the_array smallintarray,
primary key (id)
);
What I would like to do is get the id and the first element i.e. at index 1 of the array. In PostgreSQL I could write select id, the_array[1] from tbl t but I don't see how I could do that with Oracle. I read that array access by index is only possible in PL/SQL, which would be fine if I could return a "decorated cursor" to achieve the same result through JDBC, but I don't know if that's possible.
DECLARE
c1 SYS_REFCURSOR;
varr smallintarray2;
BEGIN
OPEN c1 FOR SELECT t.id, t.THE_ARRAY from tbl t;
-- SELECT t.THE_ARRAY INTO varr FROM table_with_enum_arrays2 t;
-- return a "decorated cursor" with varr(1) at select item position 1
dbms_sql.return_result(c1);
END;
You can do this in plain SQL; it's not pretty, but it does work. You would prefer that Oracle had syntax to hide this from the programmer (and perhaps it does, at least in the most recent versions; I am still stuck at 12.2).
select t.id, q.array_element
from tbl t cross apply
( select column_value as array_element,
rownum as ord
from table(the_array)
) q
where ord = 1
;
EDIT If order of generating the elements through the table operator is a concern, you could do something like this (in Oracle 12.1 and higher; otherwise the function can't be part of the query itself, but it can be defined on its own):
with
function select_element(arr smallintarray, i integer)
return number
as
begin
return arr(i);
end;
select id, select_element(the_array, 1) as the_array_1
from tbl
/
First of all, please don't do that on production. Use tables instead of storing arrays within a table.
Answer to your question is to use column as a table source
SELECT t.id, ta.*
from tbl t,
table(t.THE_ARRAY) ta
order by column_value
-- offset 1 row -- in case if sometime you'll need to skip a row
fetch first 1 row only;
UPD: as for ordering the array I can only say playing with 2asc/desc" parameters provided me with results I've expected - it has been ordered ascending or descending.
UPD2: found a cool link to description of performance issues might happen
Currently I have in my pl/sql code following statements:
-- vList looks like '1,2,3,4'
vStatement := 'SELECT NAME FROM T_USER WHERE ID IN ( ' || vList || ' ) ';
Execute Immediate vStatement BULK COLLECT INTO tNames;
I think that concatenating of query if bad practice, so I want to make this query without using stings. What is the way to rewrite this ?
P.S. maybe people here can point out why concatenation of queries is bad, because i don't have enough reasons to prove that this style is bad.
my guess is that you took some steps previously to get vList id's into a delimited string (you don't say how vList was populated ). Why not keep as one query?
begin
...
select name
bulk collect into tNames
from t_user
where id in (select id from some_table where ...);
...
Context switching when run many times can be painful, but to me the worst part is that you are blindly accepting parameter input to be a list of numbers, when it could be anything really. It could (innocently) be '1,2,X', and you'll get a runtime error "invalid number". Or worse, it could be a SQL injection attack. Its bad practice in general (dynamic sql does have its place), but definitely NOT how you're using it.
Try something like this:
create or replace type t_num_tab as table of number;
create or replace procedure test_proc(i_list in t_num_tab) as
type t_name_tab is table of varchar2(100);
l_names t_name_tab;
begin
-- get names
select name
bulk collect into l_names
from user_table
where id in (select * from table(i_list));
-- do something with l_names
dbms_output.put_line('Name count: ' || l_names.count);
end;
You can create an object type if you need something more complicated than a list of numbers.
It's not just that concatenation is slow. It's that dynamic queries in plsql are REALLY slow. Here's a good writeup of both the how and why to do this:
Ask Tom: How can I do a variable "in list"
I'm sure this is something simple, but I'm really new to PL/SQL and this has me stuck.
I've written a simple stored procedure to return a few values about a customer. Right off the bat, the %rowtype's are not coming up as reserved keywords but the compiler isn't flagging those as errors.
It is, however, ignoring the entire SQL statement flagging the line FROM demo_customers as too many values. Even if I try reducing it to only select one column it still gives me the same error.
create or replace
PROCEDURE GETCUSTOMER
(
arg_customerID demo_customers.customer_id%type,
returnRec OUT demo_customers%rowtype
)
AS
BEGIN
SELECT customer_id, cust_first_name, cust_last_name, cust_email
INTO returnRec
FROM demo_customers
WHERE customer_id = arg_customerID ;
END GETCUSTOMER;
If you want to select into a %ROWTYPE record, you'll want to do a SELECT * rather than selecting individual columns
create or replace
PROCEDURE GETCUSTOMER
(
arg_customerID demo_customers.customer_id%type,
returnRec OUT demo_customers%rowtype
)
AS
BEGIN
SELECT *
INTO returnRec
FROM demo_customers
WHERE customer_id = arg_customerID ;
END GETCUSTOMER;
If you select 4 columns explicitly, Oracle expects you to have 4 variables to select those values into.
I have a stored procedure that is doing a two-step query. The first step is to gather a list of VARCHAR2 type characters from a table and collect them into a table variable, defined like this:
TYPE t_cids IS TABLE OF VARCHAR2(50) INDEX BY PLS_INTEGER;
v_cids t_cids;
So basically I have:
SELECT item BULK COLLECT INTO v_cids FROM table_one;
This works fine up until the next bit.
Now I want to use that collection in the where clause of another query within the same procedure, like so:
SELECT * FROM table_two WHERE cid IN v_cids;
Is there a way to do this? I am able to select an individual element, but I would like to use the table variable like a would use a regular table. I've tried variations using nested selects, but that doesn't seem to work either.
Thanks a lot,
Zach
You have several choices as to how you achieve this.
If you want to use a collection, then you can use the TABLE function to select from it but the type of collection you use becomes important.
for a brief example, this creates a database type that is a table of numbers:
CREATE TYPE number_tab AS TABLE OF NUMBER
/
Type created.
The next block then populates the collection and performs a rudimentary select from it using it as a table and joining it to the EMP table (with some output so you can see what's happening):
DECLARE
-- Create a variable and initialise it
v_num_tab number_tab := number_tab();
--
-- This is a collection for showing the output
TYPE v_emp_tabtype IS TABLE OF emp%ROWTYPE
INDEX BY PLS_INTEGER;
v_emp_tab v_emp_tabtype;
BEGIN
-- Populate the number_tab collection
v_num_tab.extend(2);
v_num_tab(1) := 7788;
v_num_tab(2) := 7902;
--
-- Show output to prove it is populated
FOR i IN 1 .. v_num_tab.COUNT
LOOP
dbms_output.put_line(v_num_tab(i));
END LOOP;
--
-- Perform a select using the collection as a table
SELECT e.*
BULK COLLECT INTO v_emp_tab
FROM emp e
INNER JOIN TABLE(v_num_tab) nt
ON (e.empno = nt.column_value);
--
-- Display the select output
FOR i IN 1 .. v_emp_tab.COUNT
LOOP
dbms_output.put_line(v_emp_tab(i).empno||' is a '||v_emp_tab(i).job);
END LOOP;
END;
You can see from this that the database TYPE collection (number_tab) was treated as a table and could be used as such.
Another option would be to simply join your two tables you are selecting from in your example:
SELECT tt.*
FROM table_two tt
INNER JOIN table_one to
ON (to.item = tt.cid);
There are other ways of doing this but the first might suit your needs best.
Hope this helps.
--Doesn't work.
--SELECT item BULK COLLECT AS 'mySelectedItems' INTO v_cids FROM table_one;
SELECT table_two.*
FROM table_two INNER JOIN v_cids
ON table_two.paramname = v_cids.mySelectedItems;
Unless I'm misunderstanding the question, this should only return results that are in the table variable.
Note: I've never used Oracle, but I imagine this case would be the same.