Question: I have MS SQL function `DATEDIFF`

```
SELECT DATEDIFF(QQ, 0, '2018-09-05')

```

that returns 474(integer). 

What is PostgreSQL equivalent of this function ?

Answer: 
I think the current accepted is incorrect.

e.g. if you change date '1900-01-01' to '2017-07-01' you actually get **6** quarter diff. 

* But expected result should be:
	+ include first and last quarter: 5
	+ exclude first and last quarter: 3
	+ exclude last quarter: 4

```
    select 
        age('2018-09-05','2017-07-01') age,
        (EXTRACT(QUARTER FROM TIMESTAMP '2018-09-05') + 
      date_part('year',age('2018-09-05','2017-07-01')) * 4) -1 QUARTER

```

Result:

```
age                 |quarter|
--------------------|-------|
1 year 2 mons 4 days|      6|

```

The reason is `age('2018-09-05','2017-07-01')` return `1 year 2 mons 4 days` (maybe more than 12 month).

---

The answer i use to calculate number of quarter: 

```
with params as (
    select 
        '2017-07-01 00:00:00'::timestamp date_from,
        '2018-09-05 00:00:00'::timestamp date_to
)
select      
    extract( quarter from date_from)::int date_from_quarter,
    (extract( quarter from date_to))::int date_to_quarter,
    age(date_to, date_from) date_diff,
    (extract(year from date_to) - extract(year from date_from))::int how_many_yr,
    -- exclude begin and end
    (extract(year from date_to) - extract(year from date_from))::int* 4 - extract( quarter from date_from)::int + (extract( quarter from date_to))::int - 1 quarter_diff,
    -- exclude begin quarter 
    (extract(year from date_to) - extract(year from date_from))::int* 4 - extract( quarter from date_from)::int + (extract( quarter from date_to))::int quarter_diff_include_current_quarter
    from params
;

```

Result: 

```
date_from_quarter|date_to_quarter|date_diff           |how_many_yr|quarter_diff|quarter_diff_include_current_quarter|
-----------------|---------------|--------------------|-----------|------------|------------------------------------|
                3|              3|1 year 2 mons 4 days|          1|           3|                                   4|

```
Let's say you have two dates in the example table: start\_date and end\_date

Here is how to get the difference between these dates in number of quarters:

```
select * (year(end_date)*4 + quarter(end_date)) - (year(start_date)*4 + quarter(start_date)) as quarter_diff 
from example

```