I work with databases probably more than the next guy. Along those lines I spent a few minutes to compose a list of functions that I think are a healthy starting point for “core functionality.” There are some things so basic I did not bother to include them (like trim). I think the list below can get the average developer pretty good mileage and it would be in the interests of most database vendors to support this list.
greatest
leasttimestamp_to_epoch
time_to_epoch
date_to_epoch
epoch_to_date
epoch_to_time
epoch_to_timestampto_timestamp(value,format)
to_char(value,format)
to_date(value,format)
to_time(value,format)bitwise_shift_left
bitwise_shift_right
bitwise_and
bitwise_or
bitwise_xor
bitwise_notascii
chrquote_literal
quote_identrepeat
to_hex
from_hex
to_base64
from_base64get_byte
set_byte
get_bit
set_bitmd5
sprintf
translate
replacesleep
row_to_json
json_to_rowget_json_object
set_json_objectrow_to_xml
xml_to_rowget_xml_object
set_xml_objectconcat
concat_wsuuid
guidif
reverse
regex_replace
regex_match
[aggregrate]
bit_and
bit_or
bit_xor
bit_not
concat
concat_ws[table function]
sequence
build_ngrams
top_ngrams
sentences
regex_split
strtok_split
string_split