Dynamic pivoting with Informix SQL - sql

Question

This is my data:
date id value
1/1/2021 a 5
1/1/2021 b 10
1/1/2021 c 7
1/1/2021 d 5
1/1/2021 e 6
1/2/2021 a 4
1/2/2021 b 8
1/2/2021 c 12
1/2/2021 d 3
1/2/2021 e 5
What I want to get is this:
> 1/1/2021 1/2/2021
> a 5 4
> b 10 8
> c 7 12
> d 5 3
> e 6 5
I found soultion how to do this if date column is fixed, but it isn't. It can have other values next time. Also, I found some solutions with dynamic sql, but none of these works with Informix (at least I wasn't able to replicate those result).
How can this be done in Informix?

Answer 1

You can use dynamic SQL — or text manipulation of SQL results — to build a moderately complex SQL statement that returns the data you are after.
The answer below assumes that the table name is Data and that there is a primary key (unique) constraint on the combination of the date and id columns — assumptions that address the questions in my comment:
How many dates might you be working with? You show 2, but is it just 2 or could it be 7, 31, 365, …? Do you always have all 5 of the ID entries a .. e for each date? Is there ever any repetition of the ID values on a given date?
and answers in your response:
I don't know how many dates I might be working with, but probably from 2 to 12, shouldn't be more than 12 dates. ID's will vary too, and some dates might have them all, others don't.
Note: Informix allows you to create this table:
CREATE TABLE data
(
date DATE NOT NULL,
id CHAR(1) NOT NULL,
value INTEGER NOT NULL,
PRIMARY KEY(DATE, id)
);
Many DBMS would require the date column name to be presented as a delimited identifier enclosed in double quotes (and case-sensitive — "date"), or use a proprietary extension such as enclosing the identifier in square brackets ([date]), both in the CREATE TABLE statement and in the subsequent SQL. Informix does not — and manages to distinguish between the letters DATE as column name, data type and function name correctly.
This answer uses what I call TDQD — Test-Driven Query Design.
Relevant dates
SELECT UNIQUE date FROM data
This gives you the dates that will appear as columns. It is probable that you'll filter the data more — such as:
SELECT UNIQUE date
FROM data
WHERE date BETWEEN (TODAY - 7) AND (TODAY - 1)
ORDER BY date
You might format the results to give string usable as a column name (and using a different date range):
SELECT UNIQUE
date AS column_date,
TO_CHAR(date, 'd%Y_%m_%d') AS column_name
FROM data
WHERE date BETWEEN DATE('2021-01-01') AND DATE('2021-01-31')
ORDER BY column_date
This assumes you have set the Informix-specific environment variable DBDATE="Y4MD-" so that DATE values are presented and interpreted like DATETIME YEAR TO DAY values are.
Relevant ID values
SELECT UNIQUE id
FROM data
WHERE date BETWEEN DATE('2021-01-01') AND DATE('2021-01-31')
ORDER BY id
This will give you the list of ID values in column 1 of the final result. However, it isn't crucial to the generated SQL.
Generate SQL for Result Table
SELECT id,
MAX(CASE WHEN date = DATE('2021-01-01') THEN value ELSE NULL END) AS d2021_01_01,
MAX(CASE WHEN date = DATE('2021-01-02') THEN value ELSE NULL END) AS d2021_01_02,
MAX(CASE WHEN date = DATE('2021-01-03') THEN value ELSE NULL END) AS d2021_01_03,
MAX(CASE WHEN date = DATE('2021-01-04') THEN value ELSE NULL END) AS d2021_01_04,
MAX(CASE WHEN date = DATE('2021-01-05') THEN value ELSE NULL END) AS d2021_01_05,
MAX(CASE WHEN date = DATE('2021-01-06') THEN value ELSE NULL END) AS d2021_01_06,
MAX(CASE WHEN date = DATE('2021-01-07') THEN value ELSE NULL END) AS d2021_01_07,
MAX(CASE WHEN date = DATE('2021-01-08') THEN value ELSE NULL END) AS d2021_01_08
FROM data
GROUP BY id
ORDER BY id;
This SQL is built using the column date and column name values from the 'relevant dates' query to generate the MAX(CASE … END) AS dYYYY_MM_DD clauses in the select-list. That has to be done outside SQL — using some program to read the relevant date information and produce the corresponding SQL.
For example, if the output of the last 'relevant dates' query is in the file date.columns, this shell script would generate the requisite SQL:
printf "SELECT id"
while read column_date column_name
do
printf ",\n MAX(CASE WHEN date = DATE('%s') THEN value ELSE NULL END) AS %s" $column_date $column_name
done < date.columns
printf "\n FROM data\n GROUP BY id\n ORDER BY id;\n"
The only difference here is that the column for the date 2021-01-08 is omitted because the value is not selected by the SQL (not present in the date.columns file).
You can use any appropriate tools to run some SQL to generate the required list of dates and give the appropriate values for column_date and column_name and then format the data into an SQL statement as shown.
Sample Data
INSERT INTO data VALUES('2021-01-01', 'a', 5);
INSERT INTO data VALUES('2021-01-01', 'b', 10);
INSERT INTO data VALUES('2021-01-01', 'c', 7);
INSERT INTO data VALUES('2021-01-01', 'd', 5);
INSERT INTO data VALUES('2021-01-01', 'e', 6);
INSERT INTO data VALUES('2021-01-02', 'a', 4);
INSERT INTO data VALUES('2021-01-02', 'b', 8);
INSERT INTO data VALUES('2021-01-02', 'c', 12);
INSERT INTO data VALUES('2021-01-02', 'd', 3);
INSERT INTO data VALUES('2021-01-02', 'e', 5);
INSERT INTO data VALUES('2021-01-03', 'b', 18);
INSERT INTO data VALUES('2021-01-03', 'c', 112);
INSERT INTO data VALUES('2021-01-03', 'd', 13);
INSERT INTO data VALUES('2021-01-03', 'e', 15);
INSERT INTO data VALUES('2021-01-04', 'a', 24);
INSERT INTO data VALUES('2021-01-04', 'c', 212);
INSERT INTO data VALUES('2021-01-04', 'd', 23);
INSERT INTO data VALUES('2021-01-04', 'e', 25);
INSERT INTO data VALUES('2021-01-05', 'a', 34);
INSERT INTO data VALUES('2021-01-05', 'b', 38);
INSERT INTO data VALUES('2021-01-05', 'd', 33);
INSERT INTO data VALUES('2021-01-05', 'e', 35);
INSERT INTO data VALUES('2021-01-06', 'a', 44);
INSERT INTO data VALUES('2021-01-06', 'b', 48);
INSERT INTO data VALUES('2021-01-06', 'c', 412);
INSERT INTO data VALUES('2021-01-06', 'e', 45);
INSERT INTO data VALUES('2021-01-07', 'a', 54);
INSERT INTO data VALUES('2021-01-07', 'c', 512);
INSERT INTO data VALUES('2021-01-07', 'd', 53);
Sample output
Using a Stack Overflow Markdown table:
id
d2021_01_01
d2021_01_02
d2021_01_03
d2021_01_04
d2021_01_05
d2021_01_06
d2021_01_07
d2021_01_08
CHAR(1)
INTEGER
INTEGER
INTEGER
INTEGER
INTEGER
INTEGER
INTEGER
INTEGER
a
5
4
24
34
44
54
b
10
8
18
38
48
c
7
12
112
212
412
512
d
5
3
13
23
33
53
e
6
5
15
25
35
45
Tested on a MacBook Pro running macOS 10.14.6 Mojave (yes, antique), using IBM Informix Dynamic Server Version 12.10.FC6 (yes, also antique).