Lisp should be good for making a working SQL-alike. I can use bottom up design to abstract myself from list access to SQL's set like operators and I can get some practice using macros to replicate SQL's 3i syntax. And it'll give me something to do over the holidays, of course I'm currently unemployed so every day is a holiday! ;(
Layer 1: Basic database structure.
What does a database, table, column, row look like? How can I get tables from a database, columns or rows from a table, fields from a row etc. Here's what I decided on (in my own unique BNF that may or may not bear any resemblance to real BNF)
database -> (database (table*))
table -> (table (column*) (row*))
column -> (column column-name column-number)
row -> (row (datum*))
At this stage I can create items and get at their contents but other than that they are in the same table there is no relation between rows and columns.
Layer 2a: Adding/Removing items.
This layer just adds/removes tables to databases and columns and rows to tables.
It also maintains some basic data consistency like database, table and column name uniqueness and that rows have the correct number of elements for the tables they are added to.
I'm feeling pretty good about this because it allows me to do something that normal database systems generally can't: add and remove columns from tables in a live database while maintaining data consistency.
Layer 2b: Getting and setting data.
Using only table column and row objects (or their unique names) I can get and set at each data item uniquely.
Layer 3: Proto-SQL.
This is where things start to get interesting. I decided to make the SELECT statement and the row eliminator (WHERE clause functionality) return a temporary table. This seems like it could be pretty horrible for performance but it makes my life very easy because all the other operators can accept a temporary table just as easily as one that's connected to a database.
This layer also has the first macro the so called parse-where and my first ever use of eval-when for its sidekick expand-where.
This is also where the first major limitation occurs, consider:
WHERE (string= "COLUMN1" "VALUE1")
In normal SQL column names are double quoted and strings are single quoted. You can't do this (easily) in lisp because of #'quote/' so no database value can have the same value as a database, table or column name. I do plan to get around this using symbols at some point but not right now.
Layer 4: Real SQL-like.
This part is all macros that re-arrange, de-reference and error check the 4 implemented statements: SELECT, INSERT, UPDATE and DELETE-FROM (delete's already taken).
There are some real limitations: no order by, no joins, no sub-selects and some departures from SQL that I think make it better: where clauses are lisp expressions and the set clause for an update statement is a list of column-names and new values.
(defun test ()
(setf *databases* nil)
(let* ((database (make-database "DB1")))
(add-table (make-table "TABLE1") database)
(add-column (make-column "COLUMN0") (get-table-by-name database "TABLE1"))
(add-column (make-column "COLUMN1") (get-table-by-name database "TABLE1"))
(add-column (make-column "COLUMN2") (get-table-by-name database "TABLE1"))
(setq *current-db* database)
(insert INTO "TABLE1" VALUES ("INS00" "INS01" "INS02"))
(insert INTO "TABLE1" VALUES ("INS10" "INS11" "INS12"))
(insert INTO "TABLE1" VALUES ("INS20" "INS21" "INS22"))
(insert INTO "TABLE1" VALUES ("INS30" "INS31" "INS32"))
(insert INTO "TABLE1" VALUES ("INS40" "INS41" "INS42"))
(insert INTO "TABLE1" VALUES ("INS50" "INS51" "INS52"))
(insert INTO "TABLE1"("COLUMN0" "COLUMN1") VALUES ("INS0" "INS1"))
(insert INTO "TABLE1"("COLUMN1" "COLUMN2") VALUES ("INS1" "INS2"))
(insert INTO "TABLE1"("COLUMN0" "COLUMN2") VALUES ("INS0" "INS2"))
(print (select ("COLUMN1" "COLUMN2") from "TABLE1"))
(print (select ("COLUMN1" "COLUMN2") from "TABLE1" where
(or (string= "COLUMN0" "INS0")
(print (select * from "TABLE1"))
(select ("COLUMN0") INTO (var) FROM "TABLE1"
WHERE (string= "COLUMN0" "INS50"))
(update "TABLE1" SET (("COLUMN1" "UPD1")
WHERE (not (or (string= "COLUMN0" "INS0")
(string= "COLUMN0" "INS00")
(string= "COLUMN0" "INS10")
(string= "COLUMN0" "INS20"))))
(delete-from "TABLE1" WHERE (null "COLUMN0"))
I think that for three days work its not too shabby.
I also think that the name's pretty cool LooSeQL: Lisp SQL, loose because it's not only loosely SQL and also its loosely typed and finally I hear SQL refered to as 'sequel' a lot, I think 'squil' would be more accurate, so SeQL is better because we can all agree that it sounds like 'sequel'. It would be better if I used CLOS then I could say that it was Lisp Object Oriented SeQL, maybe next version.
Get the full source here.