I have a table mytable that stores columns in the form of JSON strings, which contain multiple key-value pairs. Now, I want to extract only a particular value corresponding to one key.
The column that stores these strings is of varchar datatype, and is created as:
insert into mytable(empid, json_column) values (1,'{"FIRST_NAME":"TOM","LAST_NAME" :"JENKINS", "DATE_OF_JOINING" :"2021-06-10", "SALARY" :"1000" }').
As you can see, json_column is created by inserting only a string. Now, I want to do something like:
select json_column.FIRST_NAME from mytable
I just want to extract the value corresponding to key FIRST_NAME.
Though my actual table is far more complex than this example, and I cannot convert these JSON keys into different columns themselves. But, this example clearly illustrates my issue.
This needs to be done over Redshift, please help me out with any valuable suggestions.
using function json_extract_path_text of Redshift can solve this problem easily, as follows:
select json_extract_path_text(json_column, 'FIRST_NAME') from mytable;
the following function deletes all blanks in a text or varchar column and returns the modified text/varchar as an int:
select condense_and_change_to_int(number_as_text_column) from mytable;
This exact query does work.
Though my goal is to apply this function to all rows of a column in order to consistently change its values. How would I do this? Is it possible with the UPDATE-clause, or do i need to do this within a function itself? I tried the following:
UPDATE mytable
SET column_to_be_modiefied = condense_and_change_to_int(column_to_be_modiefied);
Basically i wanted to input the value of the current row, modify it and save it to the column permanantly.
I'd welcome all ideas regarding how to solve scenarios like these. I'm working with postgresql (but welcome also more general solutions).
Is it possible with an update? Well, yes and sort-of.
From your description, the input to the function is a string of some sort. The output is a number. In general, numbers should be assigned to columns with a number type. The assumption is that the column in question is a number.
However, your update should work. The result will be a string representation of the number.
After running the update, you can change the column type, with something like:
alter table mytable alter column column_to_be_modiefied int;
I'm trying to select a number of fields, one of which needs to be an array with each element of the array containing two values. Each array item needs to contain a name (character varying) and an ID (numeric). I know how to return an array of single values (using the ARRAY keyword) but I'm unsure of how to return an array of an object which in itself contains two values.
The query is something like
SELECT
t.field1,
t.field2,
ARRAY(--with each element containing two values i.e. {'TheName', 1 })
FROM MyTable t
I read that one way to do this is by selecting the values into a type and then creating an array of that type. Problem is, the rest of the function is already returning a type (which means I would then have nested types - is that OK? If so, how would you read this data back in application code - i.e. with a .Net data provider like NPGSQL?)
Any help is much appreciated.
ARRAYs can only hold elements of the same type
Your example displays a text and an integer value (no single quotes around 1). It is generally impossible to mix types in an array. To get those values into an array you have to create a composite type and then form an ARRAY of that composite type like you already mentioned yourself.
Alternatively you can use the data types json in Postgres 9.2+, jsonb in Postgres 9.4+ or hstore for key-value pairs.
Of course, you can cast the integer to text, and work with a two-dimensional text array. Consider the two syntax variants for a array input in the demo below and consult the manual on array input.
There is a limitation to overcome. If you try to aggregate an ARRAY (build from key and value) into a two-dimensional array, the aggregate function array_agg() or the ARRAY constructor error out:
ERROR: could not find array type for data type text[]
There are ways around it, though.
Aggregate key-value pairs into a 2-dimensional array
PostgreSQL 9.1 with standard_conforming_strings= on:
CREATE TEMP TABLE tbl(
id int
,txt text
,txtarr text[]
);
The column txtarr is just there to demonstrate syntax variants in the INSERT command. The third row is spiked with meta-characters:
INSERT INTO tbl VALUES
(1, 'foo', '{{1,foo1},{2,bar1},{3,baz1}}')
,(2, 'bar', ARRAY[['1','foo2'],['2','bar2'],['3','baz2']])
,(3, '}b",a{r''', '{{1,foo3},{2,bar3},{3,baz3}}'); -- txt has meta-characters
SELECT * FROM tbl;
Simple case: aggregate two integer (I use the same twice) into a two-dimensional int array:
Update: Better with custom aggregate function
With the polymorphic type anyarray it works for all base types:
CREATE AGGREGATE array_agg_mult (anyarray) (
SFUNC = array_cat
,STYPE = anyarray
,INITCOND = '{}'
);
Call:
SELECT array_agg_mult(ARRAY[ARRAY[id,id]]) AS x -- for int
,array_agg_mult(ARRAY[ARRAY[id::text,txt]]) AS y -- or text
FROM tbl;
Note the additional ARRAY[] layer to make it a multidimensional array.
Update for Postgres 9.5+
Postgres now ships a variant of array_agg() accepting array input and you can replace my custom function from above with this:
The manual:
array_agg(expression)
...
input arrays concatenated into array of one
higher dimension (inputs must all have same dimensionality, and cannot
be empty or NULL)
I suspect that without having more knowledge of your application I'm not going to be able to get you all the way to the result you need. But we can get pretty far. For starters, there is the ROW function:
# SELECT 'foo', ROW(3, 'Bob');
?column? | row
----------+---------
foo | (3,Bob)
(1 row)
So that right there lets you bundle a whole row into a cell. You could also make things more explicit by making a type for it:
# CREATE TYPE person(id INTEGER, name VARCHAR);
CREATE TYPE
# SELECT now(), row(3, 'Bob')::person;
now | row
-------------------------------+---------
2012-02-03 10:46:13.279512-07 | (3,Bob)
(1 row)
Incidentally, whenever you make a table, PostgreSQL makes a type of the same name, so if you already have a table like this you also have a type. For example:
# DROP TYPE person;
DROP TYPE
# CREATE TABLE people (id SERIAL, name VARCHAR);
NOTICE: CREATE TABLE will create implicit sequence "people_id_seq" for serial column "people.id"
CREATE TABLE
# SELECT 'foo', row(3, 'Bob')::people;
?column? | row
----------+---------
foo | (3,Bob)
(1 row)
See in the third query there I used people just like a type.
Now this is not likely to be as much help as you'd think for two reasons:
I can't find any convenient syntax for pulling data out of the nested row.
I may be missing something, but I just don't see many people using this syntax. The only example I see in the documentation is a function taking a row value as an argument and doing something with it. I don't see an example of pulling the row out of the cell and querying against parts of it. It seems like you can package the data up this way, but it's hard to deconstruct after that. You'll wind up having to make a lot of stored procedures.
Your language's PostgreSQL driver may not be able to handle row-valued data nested in a row.
I can't speak for NPGSQL, but since this is a very PostgreSQL-specific feature you're not going to find support for it in libraries that support other databases. For example, Hibernate isn't going to be able to handle fetching an object stored as a cell value in a row. I'm not even sure the JDBC would be able to give Hibernate the information usefully, so the problem could go quite deep.
So, what you're doing here is feasible provided you can live without a lot of the niceties. I would recommend against pursuing it though, because it's going to be an uphill battle the whole way, unless I'm really misinformed.
A simple way without hstore
SELECT
jsonb_agg(to_jsonb (t))
FROM (
SELECT
unnest(ARRAY ['foo', 'bar', 'baz']) AS table_name
) t
>>> [{"table_name": "foo"}, {"table_name": "bar"}, {"table_name": "baz"}]
I am running into the following problem, I am passing an array of string into Oracle SQL, and I would like to retrieve all the data where its id is in the list ...
here's what i've tried ...
OPEN O_default_values FOR
SELECT ID AS "Header",
VALUE AS "DisplayValue",
VALUE_DESC AS "DisplayText"
FROM TBL_VALUES
WHERE ID IN I_id;
I_id is an array described as follows - TYPE gl_id IS TABLE OF VARCHAR2(15) INDEX BY PLS_INTEGER;
I've been getting the "expression is of wrong type" error.
The I_id array can sometimes be as large as 600 records.
My question is, is there a way to do what i just describe, or do i need to create some sort of cursor and loop through the array?
What has been tried - creating the SQL string dynamically and then con-cat the values to the end of the SQL string and then execute it. This will work for small amount of data and the size of the string is static, which will caused some other errors (like index out of range).
have a look at this link: http://asktom.oracle.com/pls/asktom/f?p=100:11:620533477655526::::P11_QUESTION_ID:139812348065
effectively what you want is a variable in-list with bind variables.
do note this:
"the" is deprecated. no need for it
today.
TABLE is it's replacement
select * from TABLE( function );
since you already have the type, all you need to do is something similar to below:
OPEN O_default_values FOR
SELECT ID AS "Header",
VALUE AS "DisplayValue",
VALUE_DESC AS "DisplayText"
FROM TBL_VALUES
WHERE ID IN (select column_value form table(I_id));
I created a table with this schema using sqlite3:
CREATE TABLE monitored_files (file_id INTEGER PRIMARY KEY,file_name VARCHAR(32767),original_relative_dir_path VARCHAR(32767),backupped_relative_dir_path VARCHAR(32767),directory_id INTEGER);
now, I would like to get all the records where original_relative_dir_path is exactly equal to '.', without 's. What I did is this:
select * from monitored_files where original_relative_dir_path='.';
The result is no records even if in the table I have just this record:
1|'P9040479.JPG'|'.'|'.'|1
I read on the web and I see no mistakes in my syntax... I also tried using LIKE '.', but still no results. I'm not an expert of SQL so maybe you can see something wrong?
Thanks!
I see no problem with the statement.
I created the table that you described.
Did an INSERT with the same values that you provided.
And did the query, and also queried without a where clause.
No problems encountered, so I suspect that when you execute your selection, you may not be connected to the correct database.