Transform SQL insert script into CSV format

Try this with GNU grep and sed:

grep -oP '\(\K[^)]*(?=\);)' file | sed 's/NULL//g;s/ //g'

Output from all four lines:

1,'asd',923123123,'zx'
1,,923123123,'zxz'
3,'asd3',923123123,
1,'asd',923123123,'zx'

or only with GNU sed:

sed 's/.*(\([^)]*\));/\1/;s/NULL//g;s/ //g' file

Output from all four lines:

1,'asd',923123123,'zx'
1,,923123123,'zxz'
3,'asd3',923123123,
1,'asd',923123123,'zx'

$ awk -F' *[(),]+ *' -v OFS=, '{for (i=2;i<NF;i++) printf "%s%s", ($i=="NULL"?"":$i), (i<(NF-1)?OFS:ORS)}' file
1,'asd',923123123,'zx'
1,,923123123,'zxz'
3,'asd3',923123123,

I'd recommend you test all potential solutions with this input:

$ cat file
INSERT INTO tbl VALUES (1, NULL, 923123123, 'foo NULL bar');

$ awk -F' *[(),]+ *' -v OFS=, '{for (i=2;i<NF;i++) printf "%s%s", ($i=="NULL"?"":$i), (i<(NF-1)?OFS:ORS)}' file
1,,923123123,'foo NULL bar'

to make sure the string NULL and blank chars are not deleted when they appear as part of a literal string.

awk -F'[()]' -v OFS=, '{gsub(/NULL|;/,"")}{gsub(/, /,",")}{print $(NF -1)}' file
1,'asd',923123123,'zx'
1,,923123123,'zxz'
3,'asd3',923123123,