SQL_Bridge: An Erlang adapter library for SQL databases (MySQL, Postgres)
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An Erlang SQL Abstraction layer for interfacing with SQL databases.

Supported Databases:

* PostgreSQL

Planned for support

* Microsoft SQL Server
* SQLite

What it does

It provides a simple layer on top of existing SQL drivers, abstracting away the pool management, and providing sensible default behaviors. Further, it provides a whole slew of methods for returning and updating data: lists, tuples, proplists, dicts, existence checks, single fields returning and setting, updating data from proplists, and more.


  • A process tracks which database it's connecting to, grabbing any of the available database pools. No need to identify a specific pool with requests.
  • Connections are made automatically when the first query is attempted
  • A process determines which database to connect to via the sql_bridge application variable lookup.
  • This is used commonly to simplify the process of running a single codebase which connects to different database for different users or for different host headers.



Probably the most unusual configuration to discuss is the module_alias.

This configuration option allows us to compile a module which exports all the functions of sql_bridge, but allows us to use a different module name.

In my own apps, I tend to use a module called db which serves as the alias to sql_bridge. While it's easy enough to make your own module and do an import, this configuration parameter allows us to skip that step.

This alias module is generated and loaded during the sql_bridge:start() function.


The most important configuration variable is the lookup variable. This tells SQL_Bridge which database to use at any given time.

It can take two possible values:

  • An atom of the database name. For single-database apps, this is the simple solution: Whatever value you assign to lookup will be the database SQL_Bridge uses.

  • A {Module, Function} or {Module, Function, Args} tuple. This is for multi-database apps. The return value of Module:Function() or erlang:apply(Module, Function, Args) will be ysed to determine which database to connect to. This value will then be cached within the process dictionary so that that (potentially expensive) function isn't repeatedly called within the same process.


As the final important config variable, adapter determines which database driver to use. Currently, SQL_Bridge ships with the following adapters (And the driver upon which it depends)

  • sql_bridge_epgsql - epgsql - The Erlang PostgreSQL driver (also uses poolboy).
  • sql_bridge_mysql_otp - mysql-otp - A New MySQL driver (also uses poolboy).
  • sql_bridge_emysql - emysql - The (mostly abandoned) Emysql driver.

Sample Config

There is a sample config file in sample.config, but here are the configuration settings currently available:

    {sql_bridge, [
        %% module_alias creates a module with the specified name, which can be
        %% used as an alias to the sigma_sql module.
        {module_alias, db},

        %% There are three adapters that can be used:
        %%  + sql_bridge_epgsql: PostgreSQL adapter using epgsql
        %%  + sql_bridge_mysql_otp: MySQL adapter using mysql-otp
        %%  + sql_bridge_emysql: MySQL adapter using the (mostly abandoned) Emysql driver.
        {adapter, sql_bridge_mysql_otp},

        %% connection parameters (self explanitory, I hope)
        {host, ""},
        {port, 3306},
        {user, "user"},
        {pass, "userpass"},

        %% There are two different ways to determine database
        %% 1) All requests go to a single database, called 'database_name':
        {lookup, database_name}
		%% 2) Before a request is made, run the function
        %% `lookup_module:lookup_function()`, for which the return value will
        %% be the database name
        {lookup, {lookup_module, lookup_function}},

		%% Number of connections to establish per pool (which really means
		%% number of connections per database).
		{connections_per_pool, 10},

        %% If a connection pool is saturated, this allows additional "overflow"
        %% connections to be established up to the limit specified below.
        {overflow_connections_per_pool, 10},

        %% If you prefer your string types (varchars, texts, etc) to be returned as erlang lists rather than binaries, set the following to true:
        {stringify_binaries, false}

The most complicated us the lookup application variable. Lookup can be one of three different kinds of values:

  • Atom: That's the database every request will use.
  • {Module, Function}: Call Module:Function() to determine which database to connect to.




Due to my obsession with brevity, most all function calls have hyper-terse versions which are acronyms of something. Learning those conventions will save you keystrokes, minimize the chance for typos, and shorten your code. The drawback is that it's not entirely obvious on a cursory glance what a function returns (for example: db:fffr is not exactly obvious that it stands for "(F)irst (F)ield of (F)irst (R)ecord").

But you'll learn common shortcuts:

  • q -> query
  • t -> tuple
  • d -> dict
  • l -> list
  • pl -> proplist
  • ff -> first field
  • fr -> first record
  • i -> insert
  • u -> update

Conveniently, however, There are also simpler, more semantic function names, like list, maps, proplist, etc, which return exactly what the name implies. All is documented below.

Prepared Statements?

SQL_Bridge currently does not offer prepared statements, but will do safe variable replacement using a similar convention, either with MySQL's ? placeholder, or PostgreSQL's $1, $2,...$X placeholder.

Replacement Placeholders

Which placeholder is used can be modified by the configuration variable replacement_token_style. This value can be the atoms 'mysql' or 'postgres' or it could also be the shortened version with the atom '?' or '$' respectively.

Sample MySQL Placeholders:

db:q("Select * from login where username=? or email=?", [Loginid, Email])

Sample PostgreSQL Placeholders:

db:q("Select * from login where username=$1 or email=$2", [Loginid, Email])

Singular Table Names

I know it's common for database designers to name their tables with the plural form of a noun to indicate that it's a collection of things (e.g. "logins" instead of "login"), while still using the singular as the name of key fields (so a table of logins would be called "logins", but the key would be "loginid").

Well, despite english being my native language, I find it to be a terribly, horribly inconsistent language, and refuse to try to make code figure out if the plural of "child" is "children" or "childs". As such, SQL_Bridge makes a lot of assumptions that your codebase will use a singular table name, and that the key of that table is named Tablename ++ "id".

Using the example above, my table of logins would be called "login" and the primary key is "loginid".

Insert or Update Determination

There are some helper functions in SQL_Bridge that will attempt to determine if we're updating or inserting a new one. The basic rule is this: If the key field specified has a value of 0 or undefined, it will be an insert, assuming the database will do the auto increment for us. If it's anything else, it's an update.


One more convention before showing each function:

Almost all query functions in SQL_Bridge take one or two parameters.

  • 1 Argument: the query will be executed as-is. (e.g. db:q("select * from whatever"))
  • 2 Arguments: Argument two should be a list of arguments that correspond to and will replace question marks (?) within the query itself in order. (e.g. db:q("select * from whatever where field1=? or field1=?", [SomeValue, SomeOtherValue]))

Table Structure for our examples

For our example, we're going to have a table called player:

| Field    | Type                              | Null | Key | Default | Extra          |
| playerid | int(10) unsigned                  | NO   | PRI | NULL    | auto_increment |
| name     | varchar(40)                       | YES  |     | NULL    |                |
| race     | enum('dwarf','orc','elf')         | YES  |     | NULL    |                |
| class    | enum('wizard','archer','bruiser') | YES  |     | NULL    |                |
| level    | int(10) unsigned                  | YES  |     | 1       |                |
| alive    | tinyint(1)                        | NO   |     | 1       |                |

Select Queries

Multi-record Queries
  • db:lists or db:q: The most basic query. Will return a list of rows formatted as simple lists.

    > db:q("select playerid, name from player where race=?", ["elf"]).
  • db:tuples or db:tq: Like db:q except returns a list of rows formatted as tuples.

    > db:tq("select playerid, name from player where race=?", ["elf"]).
  • db:proplists or db:plq: Like db:q except returns a list of proplists, with the keys of which are atomized versions of the database field names:

    > db:plq("select name, race, level from player where alive=?",[false]).
  • db:dicts or db:dq: Like db:plq, except returns a list of Erlang dicts with the keys again being atomized versions of the field names.

  • db:maps or db:mq: Like db:plq, except returns a list of Erlang maps, with keys atomized versions of field names.

Single-record Queries

Single-record queries correspond directly to their multi-record queries, except they only return a single row. They all start with fr for "first record"

  • db:list or db:fr: Like db:q (Returns a list)
  • db:tuple or db:tfr: Like db:tq (Returns a tuple)
  • db:proplist or db:plfr: Like db:plq (Returns a proplist)
  • db:dict or db:dfr: Like db:dq (Returns a dict)
  • db:map or db:mfr: List db:mq (Returns a map)
Other convenience queries
  • db:fffr: (F)irst (F)ield of (F)irst (R)ecord. Returns the first field of the first record returned.

    > db:fffr("select count(*) from player where class=?",[wizard]).
  • db:ffl: (F)irst (F)ield (L)ist. Returns a list of the first field from each row.

    > db:ffl("select playerid from player where alive=? or class=?",[true,wizard]).
  • db:qexists: Returns true or false depending on whether or not the query returns any records.

    > db:qexists("select playerid from player where playerid=?",[999]).
  • db:exists(Table, IDField, IDValue): Returns true or false depending on whether or not a record in Table exists where the specified IDField has the value IDValue.

  • db:exists(Table, IDValue): Shortcut for db:exists(Table, Table ++ "id", IDValue)

  • db:field(Table, Field, IDValue): Returns the value of the field Field from table Table, where the TableID value is IDValue.

    > db:field(player, race, 1).

    The above is the equivilant to db:fffr("select race from player where playerid=1")

  • db:field(Table, Field, IDField, IDValue)

    Like db:field/3, except you get to specify which field you're querying for instead of assuming Table ++ "id" as the ID field.

  • db:fields(Table): Returns a list of the names of the fields of the named Table

    > db:fields(player).
    [playerid, name, race, class, level, alive]

Insert, Update, Delete Queries


  • db:qi or db:qinsert Runs the specified query and returns the insert_id


  • db:qu or db:qupdate: Run the specified query and returns the number of affected rows.

Update or Delete from a Proplist, Map, or Record

  • db:save(Table, Keyfield, Data): Run an update or insert query on the Table provided with the specified Data as the row data. Data can be either a proplist, a map, or a record (See Workering with Records below). If the value in Data associated with the Key Keyfield is a zero, or is undefined, then an insert query is performed. Otherwise, an update query is performed. Regardless of insert or update method, the return value is the value of the Keyfield - if insert, then it returns the new insert_id, and if update, the value associated with the Keyfield from Data.

  • db:save(Table, Data): Like db:save(Table, Keyfield, Data) except Keyfield is deduced with list_to_atom(atom_to_list(Table) ++ "id")

  • db:update(Table, KeyField, Data): Like save/3 but will always use an update even if the value for the keyfield is 0 or undefined.

  • db:update(Table, Data): Like save/2, but uses an update operation.

  • db:insert(Table, Data): Like save/2 but uses an insert operation. Special Note:: This does not strip out any Keyfields, it inserts the data exactly as it is, so if the key field has a value of 0, that's the value that will be inserted.

Working with Records

SQL_Bridge can work with records, however, since records are done at compile time, there are some additional steps that must be performed by you in order to accomplish this. The simplest is to use the save_record() functions:

  • db:save_record(Table, KeyField, Record, FieldList): In order to call this effectively, you must pass the return value of the built-in compile function record_info(fields, RECORDNAME) as the argument for FieldList. For example, if you have a record called #foo that is saved into the table foo_tbl you could save it like this: db:save_record(foo_tbl, fooid, FooRec, record_info(fields, foo))

  • db:save_record(Table, Record, FieldList: Like db:save/2, this will automatically determine the KeyField as list_to_atom(atom_to_list(Table) ++ "id").

Using a Record Handler

SQL_Bridge also has an option to intelligently convert records into a format SQL_Bridge can work with (namely, proplists and maps). You can do this by use a record_handler configuration option. If the record_handler configuration option is specified in the sql_bridge.config file, it will call that specified function passing the record as an option.

To use this, the value of record_handler must be a 2-tuple of the format {Module, Function}, where Module:Function is a function of arity 1 and returns a proplist or map.

The simplest example would be to make a module in your app like:


handle(Foo = #foo{}) ->
	sql_bridge_utils:record_to_proplist(Foo, record_info(fields, foo));
handle(Bar = #bar{}) ->
	sql_bridge_utils:record_to_proplists(Bar, record_info(fields, bar)).

Then, in your config, set the record_handler value as follows:

[{sql_bridge, [
	{record_handler, {my_record_handler, handle}}

Once this is done, you can pass a record as the Data argument in db:save/2-3


  • db:delete(Table, ID): Delete records from a table.

  • db:delete(Table, Field, ID): Delete records from a table.


SQL_Bridge supports transactions through two mechanisms:

  1. db:start_trans(), db:commit(), and db:rollback() - Manually initiate a transaction. Note, if you run something like db:q("BEGIN"), SQL_Bridge is not intelligent enough to determine that you're in a transaction. Please use db:start_trans().
  2. db:trans(Fun) - Mnesia-style transactions where the contents of the function are run within a transaction. Note that Fun is of arity 0 (that is, no arguments). If the function completes successfully, the queries executed will be commited, and the return value of the Fun() will be the return value of db:trans(Fun). If Fun() crashes, the transaction will be automatically rolled back, and the return value will be {error, Reason}, where Reason is information about the crash (including a stacktrace).

Misc Utilities

  • db:encode(Term): Safely escapes a data for database interaction on in a SQL query, for the backend of your choice.

  • db:encode_list(List): Takes a list of terms and safely encodes them for mysql interaction, separating them with commas.

  • db:encode64(Term)

  • db:decode64(Term): Encodes and decodes any Erlang term to base64.

  • db:limit_clause(PerPage, Page): When you're doing a query that needs pagination, sometimes you just don't want to deal with figuring out the limit clause's offset and length. In this case, you can build a simple limit clause for MySQL by passing this function which page you want to show (start at page 1), and how many items you per page you want to show. It will do the offset calculation for you and return a limit clause that can be inserted into the query.

Some Quirks

There are a number of quirks to get comfortable with when using sql_bridge:

  • Dates, times, and timestamps are returned as a string (or binary), like "2016-12-31", "23:15:46", or "2016-12-31 23:15:46". This is largely for backwards compatibility with the original implementation which returned returned dates and times as strings, and since so much of my code depends on this, it's just like it.
  • Numeric and Decimal types are returned as floats or integers. Again, this is because of some code I have that depends on it.



Copyright (c) 2013-2016 Jesse Gumm (@jessegumm)

MIT License