In the ax there is a key word
firstOnly1000
Which can be used like
select firstonly1000 <ObjectName> where <where clause>
Is there a way how to make a select of the next 1000? I was thinking about using recid and
select firstonly1000 <ObjectName> where <where clause> && Recid > maxof(firstonly1000.Recid)
but in some Ax versions Recid can be lower than zero. Is there some reasonable general way to do this for all Ax versions?
I would say no. And if you're wanting to do some sort of pagination you can use a query object (see link) or something similar to the below X++, which is pretty much the common convention.
while select salesTable
{
i++;
if (i>1000)
break;
info(strFmt("%1: %2", i, salesTable.SalesId));
}
Or
// Less often used method below. Usually for a specific purpose.
select salesTable;
while (salesTable && i < 1000)
{
i++;
info(strFmt("%1: %2", i, salesTable.SalesId));
next salesTable;
}
I would think the simplest way to accomplish what you're trying with the RecId would be to sort by RecId Asc then just track the last RecId that you selected instead of maxOf. I'd think you may get poor performance though if you just arbitrarily did it on any table.
Related
So I work with Postgres SQL, and I have a jsonb column with the following structure:
{
"Store1":[
{
"price":5.99,
"seller":"seller"
},
{
"price":56.43,
"seller":"seller"
}
],
"Store2":[
{
"price":45.65,
"seller":"seller"
},
{
"price":44.66,
"seller":"seller"
}
]
}
I have a jsonb like this for every product in the database. I want to run an SQL query that will answer the following question:
For each product, is one of the prices in this JSON is bigger/equal/smaller than X?
Basically filter the product to include only the ones who have at least one price that satisfies a mathematical condition.
How can I do it efficiently? What's the best way in Postgres to iterate a JSON like this, with a relatively complex inner structure?
Also, if I could control the way the data is structured (to an extent, I can), what changes can I do to make this query more efficient?
Thanks!
Use a json path expression:
WHERE col ## '$.*[*].price < 20'
or
WHERE col #? '$.*[*] ? (#.price < 20)'
If you need to compare to another column or make the query parameterised, you can either build the jsonpath dynamically
WHERE col ## format('$.*[*].price < %s', $1)::jsonpath
WHERE col #? format('$.*[*] ? (#.price < %s)', $1)::jsonpath
or you can use the respective function and pass variables as an object:
WHERE jsonb_path_match(col, '$.*[*].price < $limit', jsonb_build_object('limit', $1))
WHERE jsonb_path_exists(col, format('$.*[*] ? (#.price < $limit)', jsonb_build_object('limit', $1))
I admit I had to check my cheat sheet to figure out the right combination of operator and expression. Takeaways:
if a comparison operator needs to work with multiple values, it generally functions as an ANY
## does not work with ? (# …) filter expressions since they don't return a boolean,
#? does not work with predicates since they always return a value (even if it's false)
What changes can I do to make this query more efficient?
As #jjanes commented on my other answer, the jsonpath match col ## '$.*[*].price < $limit' isn't going to be fast and needs to do full table scan, at least for < and >. To make a useful index, a different approach is required. An index can only have a single value to compare with, not any number. For that, we need to change the condition from EXISTS(SELECT prices_of(col) WHERE price < $limit) to (SELECT MIN(prices_of(col))) < $limit.
With this idea it is possible to build an expression index on the result of a custom immutable function:
CREATE FUNCTION min_price(data jsonb) RETURNS float
LANGUAGE SQL
IMMUTABLE
RETURNS NULL ON NULL INPUT
RETURN (
SELECT min((offer ->> 'price')::float)
FROM jsonb_each(data) AS entries(name, store),
LATERAL jsonb_array_elements(store) AS elements(offer)
);
CREATE INDEX example_min_data_price_idx ON example (min_price(data));
which you can use as
SELECT * FROM example WHERE min_price(data) < 20;
Looking for rows with a price larger than a certain number requires a separate index on max_price(data). If you want to use the index in a JOIN with more conditions, consider making it a multi-column index.
Looking for row with a price equalling a certain number can be optimised by indexing the jsonb column and using a jsonpath:
CREATE INDEX example_data_idx ON example USING GIN (data jsonb_ops);
SELECT * FROM example WHERE data ## '$.*[*].price == 20';
SELECT * FROM example WHERE data #? '$.*[*] ? (#.price == 20)';
Unfortunately you can't use jsonb_path_ops here since that doesn't support the wildcard.
Selecting the rows from a table by (partial) key with the maximum value in a particular column is a common task in SQL. This question has some excellent answers that cover a variety of approaches to it. Unfortunately I'm struggling to replicate this in my ABAP program.
None of the commonly used approaches seem to be supported:
Joining on a subquery is not supported in syntax: SELECT * FROM X as x INNER JOIN ( SELECT ... ) AS y
Using IN for a composite key is not supported in syntax as far as I know: SELECT * FROM X WHERE (key1, key2) IN ( SELECT key1 key2 FROM ... )
Left join to itself with smaller-than comparison is not supported, outer joins only support EQ comparisons: SELECT * FROM X AS x LEFT JOIN X as xmax ON x-key1 = xmax-key1 AND x-key2 < xmax-key2 WHERE xmax-key IS INITIAL
After trying each of these solutions in turn only to discover that ABAP doesn't seem to support them and being unable to find any equivalents I'm starting to think that I'll have no choice but to dump the data of the subquery to an itab.
What is the best practice for this common programming requirement in ABAP development?
First of all, specific requirement, would give you a better answer. As it happens I bumped into this question when working on a program, that uses 3 distinct methods of pseudo-grouping, (while looking for alternatives) and ALL 3 can be used to answer your question, depending on what exactly you need to do. I'm sure there are more ways to do it.
For instance, you can pull maximum values within a group by simply selecting max( your_field ) and grouping by some fields, if that's all you need.
select bname, nation, max( date_from ) from adrp group by bname, nation. "selects highest "from" date for each bname
If you need to use that max value as a filter condition within a query, you can do it by performing pseudo-grouping using sub-query and max within sub-query like this (notice how I move out the BNAME check into sub query, which means I don't have to check both fields using in (subquery) addition):
select ... from adrp as b_adrp "Pulls the latest person info for a user (some conditions are missing, but this is a part of an actual query)
where b_adrp~date_from in (
select max( date_from ) "Highest date_from where both dates are valid
from adrp where persnumber = b_adrp~persnumber and nation = b_adrp~nation and date_from <= #sy-datum )
The query above allows you to select selects all user info from base query and (where the first one only allows to take aggregated and grouped data).
Finally, If you need to check based on composite key and compare it to multiple agregate function results, the implementation will heavily depend on specifics of your requirement (and since your question has none, I'll provide a generic one). Easiest option is to use exists / not exists instead of in (subquery), in exact same way and form the subquery to check for existance of specific key or condition rather than pull a list ( you can nest subqueries if you have to ):
select * from bkpf where exists ( select 1 from bkpf as b where belnr = bkpf~belnr and gjahr = bkpf~gjahr group by belnr, gjahr having max( budat ) = bkpf~budat ) "Took an available example, that I had in testing program.
All 3 queries will get you max value of a column within a group and in fact, all 3 can use joins to achieve identical results.
please find my answers below your questions.
Joining on a subquery is not supported in syntax: SELECT * FROM X as x INNER JOIN ( SELECT ... ) AS y
Putting the subquery in your where condition should do the work SELECT * FROM X AS x INNER JOIN Y AS y ON x~a = y~b WHERE ( SELECT * FROM y WHERE ... )
Using IN for a composite key is not supported in syntax as far as I know: SELECT * FROM X WHERE (key1, key2) IN ( SELECT key1 key2 FROM ... )
You have to split your WHERE clause: SELECT * FROM X WHERE key1 IN ( SELECT key1 FROM y ) AND key2 IN ( SELECT key2 FROM y )
Left join to itself with smaller-than comparison is not supported, outer joins only support EQ comparisons.
Yes, thats right at the moment.
Left join to itself with smaller-than comparison is not supported, outer joins only support EQ comparisons:
SELECT * FROM X AS x LEFT JOIN X as xmax ON x-key1 = xmax-key1 AND x-key2 < xmax-key2 WHERE xmax-key IS INITIAL
This is not true. This SELECT is perfectly valid:
SELECT b1~budat
INTO TABLE lt_bkpf
FROM bkpf AS b1
LEFT JOIN bkpf AS b2
ON b2~belnr < b1~belnr
WHERE b1~bukrs <> ''.
And was valid at least since 7.40 SP08, since July 2013, so at the time you asked this question it was valid as well.
When you retrieve data through a MySQLi statement such as the following:
$sqls = "SELECT * FROM course WHERE course='$product_id_array' OR course='Both' ORDER BY ...";
$sqlsresults = mysqli_query($db_conx,$sqls);
while($row = mysqli_fetch_assoc($sqlsresults)) {
$selectedContent = $row["content"];
$selectedTitle = $row["title"];
}
Is there a way to output the match for 'both' first? Would this be as simple as re-arranging the order of the WHERE portion of the statement?
Your ORDER BY explicitly sorts only by id. There is no way around this in the WHERE part of your query. To change how results are ordered, use ORDER BY.
SELECT *
FROM course
WHERE course=...
OR course='Both'
ORDER BY CASE course WHEN 'Both' THEN 0 ELSE 1 END ASC, id DESC
Unrelated note: if $product_id_array contains untrusted user input, the user can put things like '; DELETE FROM course; -- in there. Read up on parameterized queries to learn how to prevent that.
In Python, I can write a sort comparison function which returns an item in the set {-1, 0, 1} and pass it to a sort function like so:
sorted(["some","data","with","a","nonconventional","sort"], custom_function)
This code will sort the sequence according to the collation order I define in the function.
Can I do the equivalent in Postgres?
e.g.
SELECT widget FROM items ORDER BY custom_function(widget)
Edit: Examples and/or pointers to documentation are welcome.
Yes you can, you can even create an functional index to speed up the sorting.
Edit: Simple example:
CREATE TABLE foo(
id serial primary key,
bar int
);
-- create some data
INSERT INTO foo(bar) SELECT i FROM generate_series(50,70) i;
-- show the result
SELECT * FROM foo;
CREATE OR REPLACE FUNCTION my_sort(int) RETURNS int
LANGUAGE sql
AS
$$
SELECT $1 % 5; -- get the modulo (remainder)
$$;
-- lets sort!
SELECT *, my_sort(bar) FROM foo ORDER BY my_sort(bar) ASC;
-- make an index as well:
CREATE INDEX idx_my_sort ON foo ((my_sort(bar)));
The manual is full of examples how to use your own functions, just start playing with it.
SQL: http://www.postgresql.org/docs/current/static/xfunc-sql.html
PL/pgSQL: http://www.postgresql.org/docs/current/static/plpgsql.html
We can avoid confusion about ordering methods using names:
"score function" of standard SQL select * from t order by f(x) clauses, and
"compare function" ("sort function" in the question text) of the Python's sort array method.
The ORDER BY clause of PostgreSQL have 3 mechanisms to sort:
Standard, using an "score function", that you can use also with INDEX.
Special "standard string-comparison alternatives", by collation configuration (only for text, varchar, etc. datatypes).
ORDER BY ... USING clause. See this question or docs example. Example: SELECT * FROM mytable ORDER BY somecol USING ~<~ where ~<~ is an operator, that is embedding a compare function.
Perhaps "standard way" in a RDBMS (as PostgreSQL) is not like Python's standard because indexing is the aim of a RDBMS, and it's easier to index score functions.
Answers to the question:
Direct solution. There are no direct way to use an user-defined function as compare function, like in the sort method of languages like Python or Javascript.
Indirect solution. You can use a user-defined compare function in an user-defined operator, and an user-defined operator class to index it. See at PostgreSQL docs:
CREATE OPERATOR with the compare function;
CREATE OPERATOR CLASS, to be indexable.
Explaining compare functions
In Python, the compare function looks like this:
def compare(a, b):
return 1 if a > b else 0 if a == b else -1
The compare function use less CPU tham a score function. It is usefull also to express order when score funcion is unknown.
See a complete description at
for C language see https://www.gnu.org/software/libc/manual/html_node/Comparison-Functions.html
for Javascript see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort#Description
Other typical compare functions
Wikipedia's example to compare tuples:
function tupleCompare((lefta, leftb, leftc), (righta, rightb, rightc))
if lefta ≠ righta
return compare(lefta, righta)
else if leftb ≠ rightb
return compare(leftb, rightb)
else
return compare(leftc, rightc)
In Javascript:
function compare(a, b) {
if (a is less than b by some ordering criterion) {
return -1;
}
if (a is greater than b by the ordering criterion) {
return 1;
}
// a must be equal to b
return 0;
}
C++ example of PostgreSQL docs:
complex_abs_cmp_internal(Complex *a, Complex *b)
{
double amag = Mag(a),
bmag = Mag(b);
if (amag < bmag)
return -1;
if (amag > bmag)
return 1;
return 0;
}
You could do something like this
SELECT DISTINCT ON (interval_alias) *,
to_timestamp(floor((extract('epoch' FROM index.created_at) / 10)) * 10) AT
TIME ZONE 'UTC' AS interval_alias
FROM index
WHERE index.created_at >= '{start_date}'
AND index.created_at <= '{end_date}'
AND product = '{product_id}'
GROUP BY id, interval_alias
ORDER BY interval_alias;
Firstly you define the parameter that will be your ordering column with AS. It could be function or any SQL expression. Then set it to ORDER BY expression and you're done!
In my opinion, this is the smoothest way to do such an ordering.
I have a large database, that I want to do some logic to update new fields.
The primary key is id for the table harvard_assignees
The LOGIC GOES LIKE THIS
Select all of the records based on id
For each record (WHILE), if (state is NOT NULL && country is NULL), update country_out = "US" ELSE update country_out=country
I see step 1 as a PostgreSQL query and step 2 as a function. Just trying to figure out the easiest way to implement natively with the exact syntax.
====
The second function is a little more interesting, requiring (I believe) DISTINCT:
Find all DISTINCT foreign_keys (a bivariate key of pat_type,patent)
Count Records that contain that value (e.g., n=3 records have fkey "D","388585")
Update those 3 records to identify percent as 1/n (e.g., UPDATE 3 records, set percent = 1/3)
For the first one:
UPDATE
harvard_assignees
SET
country_out = (CASE
WHEN (state is NOT NULL AND country is NULL) THEN 'US'
ELSE country
END);
At first it had condition "id = ..." but I removed that because I believe you actually want to update all records.
And for the second one:
UPDATE
example_table
SET
percent = (SELECT 1/cnt FROM (SELECT count(*) AS cnt FROM example_table AS x WHERE x.fn_key_1 = example_table.fn_key_1 AND x.fn_key_2 = example_table.fn_key_2) AS tmp WHERE cnt > 0)
That one will be kind of slow though.
I'm thinking on a solution based on window functions, you may want to explore those too.