php - Insert records if not exist to specific columns with special conditions in MySQL -

i have csv file i'd insert table (let's call my_table).

for task, load csv file new temporary table called temp_table (has same features/structure my_table). values don't exist should inserted my_table there conditions must fulfilled:

if value in column letter in my_table doesn't exist then:

1. insert value in 'letter' column letter in my_table
2. also insert 'num' value column num in my_table

if value 'letter' exists, value in 'num' missing in my_table , update record in column 'num'

here example table in temp_table (left) , my_table (right):

    temp_table                my_table        letter   num            letter   num     1  aab     123            aab      123      2  aac     123            ---      ---  (both don't exist)     3  bba     234            bba      ---  (only num doesn't exist)                  result my_table:                    letter     num                   1 aab      123                   2 aac      123                      3 bba      234 

*order (id) doesn't matter.

edit: can see value in line 1 isn't inserted my_table because value aab exists. in second line, aac doesn't exist , therefore inserted value of num. in third line, bba needs updated, column num gets value.

does knows how in mysql?

or easier/make more sense try write in php?

additional: came far in order overcome part 1 -

insert my_table (letter)  select distinct letter temp_table  not exists  (select * my_table my_table.letter = temp_table.letter); 

the above mentioned job partially. inserts values not exists in letter only. part two? inserting rows of column num in rows in letter missing? in row 2 of example.. purpose tried:

insert my_table (letter, num)  select distinct letter, num temp_table  not exists  (select * my_table my_table.letter = temp_table.letter  , my_table.num = temp_table.num); 

however, didn’t work wanted. although did skip first row , indeed inserted row (2) should have:

2  aac     123 

it inserted row:

letter     num  1  aab      123  2  aac      123     3  bba      4  bba      234  

as new row, , shouldn’t have occurred! because bba exists , condition part 3.

so mentioned before, value column num should inserted, if , if row in column letter exists !!

if have unique constraint on letter can use mysql's insert...on duplicate key update syntax, little bit of logic num column:

insert my_table (letter, num)  select letter, num temp_table  on duplicate key update   num = coalesce(num, values(num));