Q:

Updating a PostgreSQL Table With Different Random Dates

I have two tables in PostgreSQL, Employee and Leave like following:  
CREATE TEMP TABLE employee_table AS SELECT
  id::int,
  name::text
FROM ( VALUES
  (1, 'John' ),
  (2, 'David')  
) AS t(id, name);

CREATE TEMP TABLE leave_table AS SELECT
  id::int,
  leave_date::date,
  emp_id::int
FROM ( VALUES
  (1, '01/10/1993' ,1),
  (2, Null         ,1),
  (3, Null         ,1),
  (4, '02/12/1990' ,2),
  (5, Null         ,2),
  (6, Null         ,2) 
) AS t(ID,Leave_Date,Emp_ID);

I want to update the Leave table and set the Leave_Date column to a random date maybe 01/01/2000 for each employee but if an employee has more than one Null entry in Leave table, I want to update his null entries with two different dates which means one employee should not have two same Leave_Date value in Leave table and after update my leave table should look like following:  
ID        Leave_Date    Emp_ID  
1         01/10/1993      1  
2         01/01/2000      1  
3         01/01/2001      1  

As shown above, initially john had two null entries in Leave table and the result shows that those entries are update with two different dates. Is there any way to this?

A:

First you never gave a test case for when there is only one date and it is null, so we create that.
INSERT INTO employee_table (id, name) VALUES (3, 'Evan Carroll');
INSERT INTO leave_table VALUES ( 10, null, 3 );

Then we run a command to check whether or not an emp_id has more than one entry in leave_table. The results are in that derived table. We update accordingly. Here we generate a date that represents the year start between 1900-2020. Just update this for what you mean by "random date" you didn't define it in your question.
UPDATE leave_table
SET leave_date = CASE
  WHEN t.count = 1 OR t.count IS NULL
  THEN '01/01/2000'::date
  ELSE '1/1/1900'::date + ('1 year'::interval*floor(random()*120))
END
FROM (
  SELECT emp_id, count(*) FROM leave_table
  WHERE leave_date IS NULL
  GROUP BY emp_id
) AS t
WHERE leave_date IS NULL
AND t.emp_id = leave_table.emp_id;

Then we have it
TABLE leave_table;
 id | leave_date | emp_id 
----+------------+--------
  1 | 1993-01-10 |      1
  4 | 1990-02-12 |      2
  2 | 1964-01-01 |      1
  3 | 1929-01-01 |      1
  5 | 1933-01-01 |      2
  6 | 1902-01-01 |      2
 10 | 2000-01-01 |      3

Now, as @McNets pointed out yesterday, I am kind of cheating. Instead, try this (much more complex query) which suffices the question's update his [emp_id] null entries with two different dates
WITH t AS (
  SELECT
    id,
    emp_id,
    leave_date,
    count(*) OVER (PARTITION BY emp_id) AS max_nulls,
    row_number() OVER (PARTITION BY emp_id)
  FROM leave_table
  WHERE leave_table.leave_date IS NULL
)
UPDATE leave_table
SET leave_date = CASE
  WHEN t.max_nulls = 1 OR t.max_nulls IS NULL
  THEN '01/01/2000'::date
  ELSE date_series_emp.ds
END
FROM t
INNER JOIN (
  SELECT distinct_emps.emp_id,
    gs.ds,
    count(*) OVER (PARTITION BY emp_id ORDER BY random()) AS row_number
  FROM ( SELECT DISTINCT emp_id FROM leave_table ) AS distinct_emps
  CROSS JOIN generate_series('1/1/1900'::date, '1/1/1990'::date, '1 month')
    AS gs(ds)
) AS date_series_emp
  USING ( emp_id, row_number )
WHERE t.id = leave_table.id;

Breaking it apart, the CTE does this
  SELECT
    id,
    emp_id,
    leave_date,
    count(*) FILTER (WHERE leave_date IS NULL) OVER (PARTITION BY emp_id) AS max_nulls,
    row_number() OVER (PARTITION BY emp_id)
  FROM leave_table

That generates how many nulls are in the set, and row numbers from within the set that we can join on for a 1:1 with the update query,
 id │ emp_id │ leave_date │ max_nulls │ row_number 
────┼────────┼────────────┼───────────┼────────────
  2 │      1 │            │         2 │          1
  3 │      1 │            │         2 │          2
  5 │      2 │            │         2 │          1
  6 │      2 │            │         2 │          2
 10 │      3 │            │         1 │          1

The only other tricky part is the inner-join select,
  SELECT distinct_emps.emp_id,
    gs.ds,
    count(*) OVER (PARTITION BY emp_id ORDER BY random()) AS row_number
  FROM ( SELECT DISTINCT emp_id FROM leave_table ) AS distinct_emps
  CROSS JOIN generate_series('1/1/1900'::date, '1/1/1990'::date, '1 month')
    AS gs(ds)

There we're taking the distinct emp_ids, and joining them on a sequence of dates that you're calling random. We count(*) over that sequence to give it a corresponding random number from within the cardinal sequences generated.
Then we join this to the table and perform the update..
This method does have a one drawback, if the input size ever exhausts your pool of "random dates" (only 1081 of them), the update on rows past that max won't be performed at all.

