How to design this table schema (and how to query it) - sql

Question

I have a table that stores a list of members - for the sake of simplicity, I will use a simple real-world case that models my use case.
Let's use the analogy of a sports club or gym.
The membership of the gym changes every three months (for example) - with some old members leaving, some new members joining and some members staying unchanged.
I want to run a query on the table - spanning a multi-time period and return the average weight of all of the members in the club.
These are the tables I have come up with so far:
-- A table containing all members the gym has ever had
-- current members have their leave_date field left at NULL
-- departed members have their leave_date field set to the days they left the gym
CREATE TABLE IF NOT EXISTS member (
id PRIMARY KEY NOT NULL,
name TEXT NOT NULL,
join_date DATE NOT NULL,
-- set to NULL if user has not left yet
leave_date DATE DEFAULT NULL
);
-- A table of members weights.
-- This table is populated DAILY,after the weights of CURRENT members
-- has been recorded
CREATE TABLE IF NOT EXISTS current_member_weight (
id PRIMARY KEY NOT NULL,
calendar_date DATE NOT NULL,
member_id INTEGER REFERENCES member(id) NOT NULL,
weight REAL NOT NULL
);
-- I want to write a query that returns the AVERAGE daily weight of
-- CURRENT members of the gym. The query should take a starting_date
-- and an ending_date between which to calculate the daily
-- averages. The aver
-- PSEUDO SQL BELOW!
SELECT calendar_date, AVG(weight)
FROM member, current_member_weight
WHERE calendar_date BETWEEN(starting_date, ending_date);
I have two questions:
can the schema above be improved - if yes, please illustrate
How can I write an SQL* query to return the average weights calculated for all members in the gym during a specified period (t1, t2), where (t1,t2) spans a period that members have joined/left the gym?
[[Note about SQL]]
Preferably, any SQL shown would be database anagnostic, however if a particular flavour of SQL is to be used, I'd prefer PostgreSQL, since that this is the database I'm using.

Answer 1

Below SQL would work as long as the data in the gym_member table is consistent with the joining and leaving date of each member (i.e. for any member, the gym_member table should not have rows with calendar_date less his joining date or with calendar_date greater than his leaving date)
SELECT
gm.calendar_date,
AVG(gm.weight) avg_weight
FROM
member m,
gym_member gm
WHERE
m.id = gm.member_id
AND
gm.calendar_date >= '1-Jan-2017'
AND
gm.calendar_date <= '31-Dec-2017'
GROUP BY
gm.calendar_date