LDAP is a stripped down version of the X.500 directory, but I wasn’t aware that had anything to do with SQL. X.500 is hierarchical, not relational, and it doesn’t have a query language as such - http://sec.cs.kent.ac.uk/x500book/Chapter.5/Chapter5b.htm#5.... - neither did the original version of LDAP (RFC 1487 https://tools.ietf.org/html/rfc1487 RFC 1488 https://tools.ietf.org/html/rfc1488). The text representation of filters didn’t turn up until RFC 1960 https://www.rfc-editor.org/rfc/rfc1960.html (1996) and it’s just a rendering of the ASN.1 structure.

I thought SQL descended relatively straightforwardly from SEQUEL.