Get previous days' value in column in postgres

Question

I have a question.

I have a sql command that is getting the moving average for each day using window functions.

        BEGIN;
            DROP TABLE IF EXISTS vol_stats;
            SELECT pk as fk, 
                   avg(CAST(volume as FLOAT)) over (partition by account_id order by "endts") as average,
            INTO vol_stats
            from volume_temp
            order by account_id, "endts";
        COMMIT;

I would like to get one more value and that is the previous days' value.

The data structure looks like this.

acccount_id | value | timestamp 
-------------------------------
a12         | 122   | jan 1
a13         | 133   | jan 1
a14         | 443   | jan 1
a12         | 251   | jan 2
a13         | 122   | jan 2
a14         | 331   | jan 2
a12         | 412   | jan 3
a13         | 323   | jan 3
a14         | 432   | jan 3

and we are computing this

acccount_id | value | timestamp | Average
-----------------------------------------
a12         |  122  | jan 1     | 122
a13         |  133  | jan 1     | 133
a14         |  443  | jan 1     | 443
a12         |  251  | jan 2     | 188.5
a13         |  122  | jan 2     | 222.5
a14         |  331  | jan 2     | 387
a12         |  412  | jan 3     | 261.6
a13         |  323  | jan 3     | 192.6
a14         |  432  | jan 3     | 402

What would be helpful would be to grab the previous days' value as well. Like this.

acccount_id | value | timestamp | Average | previous
-----------------------------------------
a12         |  122  | jan 1     | 122     | null
a13         |  133  | jan 1     | 133     | null
a14         |  443  | jan 1     | 443     | null
a12         |  251  | jan 2     | 188.5   | 122
a13         |  122  | jan 2     | 222.5   | 133
a14         |  331  | jan 2     | 387     | 443
a12         |  412  | jan 3     | 261.6   | 251
a13         |  323  | jan 3     | 192.6   | 122
a14         |  432  | jan 3     | 402     | 331

Answer 1

Just add another column to the SELECT list:

lag(volume) OVER (PARTITION BY account_id ORDER BY endts)