EmacSQL

EmacSQL is a high-level Emacs Lisp front-end for SQLite (primarily), PostgreSQL, MySQL, and potentially other SQL databases.

Any readable lisp value can be stored as a value in EmacSQL, including numbers, strings, symbols, lists, vectors, and closures. EmacSQL has no concept of "TEXT" values; it's all just lisp objects. The lisp object nil corresponds 1:1 with NULL in the database.

On MELPA, each back-end is provided as a separate package, suffixed with the database name. In the case of emacsql-sqlite, on first use EmacSQL will attempt to find a C compiler and use it to compile a custom native binary for communicating with a SQLite database.

Requires Emacs 25 or later.

FAQ

Why are all values stored as strings?

EmacSQL is not intended to interact with arbitrary databases, but to be an ACID-compliant database for Emacs extensions. This means that EmacSQL cannot be used with a regular SQL database used by other non-Emacs clients.

All database values must be s-expressions. When EmacSQL stores a value — string, symbol, cons, etc. — it is printed and written to the database in its printed form. Strings are wrapped in quotes and escaped as necessary. That means "bare" symbols in the database generally look like strings. The only exception is nil, which is stored as NULL.

Will EmacSQL ever support arbitrary databases?

The author of EmacSQL thinks that it was probably a design mistake to restrict it to Emacs by storing only printed values, and that it would be a lot more useful if it just handled primitive database types.

However, EmacSQL is in maintenance mode and there are no plans to make any fundamental changes, not least because they would break all existing packages and databases that rely on the current EmacSQL behavior.

Windows Issues

Emacs start-process-shell-command function is not supported on Windows. Since both emacsql-mysql and emacsql-psql rely on this function, neither of these connection types are supported on Windows.

Example Usage

(defvar db (emacsql-sqlite "~/company.db"))

;; Create a table. Table and column identifiers are symbols.
(emacsql db [:create-table people ([name id salary])])

;; Or optionally provide column constraints.
(emacsql db [:create-table people
             ([name (id integer :primary-key) (salary float)])])

;; Insert some data:
(emacsql db [:insert :into people
             :values (["Jeff" 1000 60000.0] ["Susan" 1001 64000.0])])

;; Query the database for results:
(emacsql db [:select [name id]
             :from people
             :where (> salary 62000)])
;; => (("Susan" 1001))

;; Queries can be templates, using $1, $2, etc.:
(emacsql db [:select [name id]
             :from people
             :where (> salary $s1)]
         50000)
;; => (("Jeff" 1000) ("Susan" 1001))

When editing these prepared SQL s-expression statements, the M-x emacsql-show-last-sql command (think eval-last-sexp) is useful for seeing what the actual SQL expression will become when compiled.

Schema

A table schema is a list whose first element is a vector of column specifications. The rest of the list specifies table constraints. A column identifier is a symbol and a column's specification can either be just this symbol or it can include constraints as a list. Because EmacSQL stores entire lisp objects as values, the only relevant (and allowed) types are integer, float, and object (default).

([(<column>) ...] (<table-constraint> ...) ...])

Dashes in identifiers are converted into underscores when compiled into SQL. This allows for lisp-style identifiers to be used in SQL. Constraints follow the compilation rules below.

;; No constraints schema with four columns:
([name id building room])

;; Add some column constraints:
([(name :unique) (id integer :primary-key) building room])

;; Add some table constraints:
([(name :unique) (id integer :primary-key) building room]
 (:unique [building room])
 (:check (> id 0)))

Here's an example using foreign keys.

;; "subjects" table schema
([(id integer :primary-key) subject])

;; "tag" table references subjects
([(subject-id integer) tag]
 (:foreign-key [subject-id] :references subjects [id]
               :on-delete :cascade))

Foreign key constraints are enabled by default in EmacSQL.

Operators

Expressions are written lisp-style, with the operator first. If it looks like an operator EmacSQL treats it like an operator. However, several operators are special.

<=    >=    funcall    quote

The <= and >= operators accept 2 or 3 operands, transforming into a SQL _ BETWEEN _ AND _ operator as appropriate.

For function-like "operators" like count and max use the funcall "operator."

[:select (funcall max age) :from people]

Inside expressions, EmacSQL cannot tell the difference between symbol literals and column references. If you're talking about the symbol itself, just quote it as you would in normal Elisp. Note that this does not "escape" $tn parameter symbols.

(emacsql db [... :where (= category 'hiking)])

Quoting a string makes EmacSQL handle it as a "raw string." These raw strings are not printed when being assembled into a query. These are intended for use in special circumstances like filenames (ATTACH) or pattern matching (LIKE). It is vital that raw strings are not returned as results.

(emacsql db [... :where (like name '"%foo%")])
(emacsql db [:attach '"/path/to/foo.db" :as foo])

Since template parameters include their type they never need to be quoted.

With glob and like SQL operators keep in mind that they're matching the printed representations of these values, even if the value is a string.

The || concatenation operator is unsupported because concatenating printed representations breaks an important constraint: all values must remain readable within SQLite.

Prepared Statements

The database is interacted with via prepared SQL s-expression statements. You shouldn't normally be concatenating strings on your own. (And it leaves out any possibility of a SQL injection!) See the "Usage" section above for examples. A statement is a vector of keywords and other lisp object.

Prepared EmacSQL s-expression statements are compiled into SQL statements. The statement compiler is memorized so that using the same statement multiple times is fast. To assist in this, the statement can act as a template -- using $i1, $s2, etc. -- working like the Elisp format function.

Compilation Rules

Rather than the typical uppercase SQL keywords, keywords in a prepared EmacSQL statement are literally just that: lisp keywords. EmacSQL only understands a very small amount of SQL's syntax. The compiler follows some simple rules to convert an s-expression into SQL.

All prepared statements are vectors.

A prepared s-expression statement is a vector beginning with a keyword followed by a series of keywords and special values. This includes most kinds of sub-queries.

[:select ... :from ...]
[:select tag :from tags
 :where (in tag [:select ...])]

Keywords are split and capitalized.

Dashes are converted into spaces and the keyword gets capitalized. For example, :if-not-exists becomes IF NOT EXISTS. How you choose to combine keywords is up to your personal taste (e.g. :drop :table vs. :drop-table).

Standalone symbols are identifiers.

EmacSQL doesn't know what symbols refer to identifiers and what symbols should be treated as values. Use quotes to mark a symbol as a value. For example, people here will be treated as an identifier.

[:insert-into people :values ...]

Row-oriented information is always represented as vectors.

This includes rows being inserted, and sets of columns in a query. If you're talking about a row-like thing then put it in a vector.

[:select [id name] :from people]

Note that * is actually a SQL keyword, so don't put it in a vector.

[:select * :from ...]

Lists are treated as expressions.

This is true even within row-oriented vectors.

[... :where (= name "Bob")]
[:select [(/ seconds 60) count] :from ...]

Some things that are traditionally keywords -- particularly those that are mixed in with expressions -- have been converted into operators (AS, ASC, DESC).

[... :order-by [(asc b), (desc a)]]   ; "ORDER BY b ASC, a DESC"
[:select p:name :from (as people p)]  ; "SELECT p.name FROM people AS p"

The :values keyword is special.

What follows :values is always treated like a vector or list of vectors. Normally this sort of thing would appear to be a column reference.

[... :values [1 2 3]]
[... :values ([1 2 3] [4 5 6])]  ; insert multiple rows

A list whose first element is a vector is a table schema.

This is to distinguish schemata from everything else. With the exception of what follows :values, nothing else is shaped like this.

[:create-table people ([(id :primary-key) name])]

Templates

To make statement compilation faster, and to avoid making you build up statements dynamically, you can insert $tn parameters in place of identifiers and values. These refer to the argument's type and its argument position after the statement in the emacsql function, one-indexed.

(emacsql db [:select * :from $i1 :where (> salary $s2)] 'employees 50000)

(emacsql db [:select * :from employees :where (like name $r1)] "%Smith%")

The letter before the number is the type.

• i : identifier
• s : scalar
• v : vector (or multiple vectors)
• r : raw, unprinted strings
• S : schema

When combined with :values, the vector type can refer to lists of rows.

(emacsql db [:insert-into favorite-characters :values $v1]
            '([0 "Calvin"] [1 "Hobbes"] [3 "Susie"]))

This is why rows must be vectors and not lists.

SQLite Support

The custom EmacSQL SQLite binary is compiled with Soundex and full-text search (FTS3, FTS4, and FTS5) enabled -- features disabled by the default SQLite build. This back-end should work on any system with a conforming ANSI C compiler installed under a command name listed in emacsql-sqlite-c-compilers.

Ignored Features

EmacSQL doesn't cover all of SQLite's features. Here are a list of things that aren't supported, and probably will never be.

• Collating. SQLite has three built-in collation functions: BINARY (default), NOCASE, and RTRIM. EmacSQL values never have right-hand whitespace, so RTRIM won't be of any use. NOCASE is broken (ASCII-only) and there's little reason to use it.

• Text manipulation functions. Like collating this is incompatible with EmacSQL s-expression storage.

• Date and time. These are incompatible with the printed values stored by EmacSQL and therefore have little use.

Limitations

EmacSQL is not intended to play well with other programs accessing the SQLite database. Non-numeric values are stored encoded as s-expressions TEXT values. This avoids ambiguities in parsing output from the command line and allows for storage of Emacs richer data types. This is an efficient, ACID-compliant database specifically for Emacs.

Emacs Lisp Indentation Annoyance

By default, emacs-lisp-mode indents vectors as if they were regular function calls.

;; Ugly indentation!
(emacsql db [:select *
                     :from people
                     :where (> age 60)])

Calling the function emacsql-fix-vector-indentation (interactive) advises the major mode to fix this annoyance.

;; Such indent!
(emacsql db [:select *
             :from people
             :where (> age 60)])

Contributing and Extending

To run the test suite, clone the pg and sqlite3 packages into sibling directories. The Makefile will automatically put these paths on the Emacs load path (override LDFLAGS if your situation is different).

git clone https://github.com/emarsden/pg-el ../pg
git clone https://github.com/pekingduck/emacs-sqlite3-api ../sqlite3

Or set LOAD_PATH to point at these packages elsewhere:

make LOAD_PATH='-L path/to/pg -L path/to/sqlite3'

Then invoke make:

make test

If the environment variable PGDATABASE is present then the unit tests will also be run with PostgreSQL (emacsql-psql). Provide PGHOST, PGPORT, and PGUSER if needed. If PGUSER is provided, the pg.el back-end (emacsql-pg) will also be tested.

If the environment variable MYSQL_DBNAME is present then the unit tests will also be run with MySQL in the named database. Note that this is not an official MySQL variable, just something made up for EmacSQL.

Creating a New Front-end

EmacSQL uses EIEIO so that interactions with a connection occur through generic functions. You need to define a new class that inherits from emacsql-connection.

• Implement emacsql-send-message, emacsql-waiting-p, emacsql-parse, and emacsql-close.
• Provide a constructor that initializes the connection and calls emacsql-register (for automatic connection cleanup).
• Provide emacsql-types if needed (hint: use a class-allocated slot).
• Ensure that you properly read NULL as nil (hint: ask your back-end to print it that way).
• Register all reserved words with emacsql-register-reserved.
• Preferably provide emacsql-reconnect if possible.
• Set the default isolation level to serializable.
• Enable autocommit mode by default.
• Prefer ANSI syntax (value escapes, identifier escapes, etc.).
• Enable foreign key constraints by default.

The goal of the autocommit, isolation, parsing, and foreign key configuration settings is to normalize the interface as much as possible. The connection's user should have the option to be agnostic about which back-end is actually in use.

The provided implementations should serve as useful examples. If your back-end outputs data in a clean, standard way you may be able to use the emacsql-protocol-mixin class to do most of the work.

See Also

• SQLite Documentation