LJSQlite3: SQlite3 Interface

SQLite databases require no preliminary setup (installation and configuration or a database server), offer competitive performance and are portable as each database is fully contained in a single file. Additionally the command-line shell can be used to facilitate common tasks. Thanks to all these features SQLite databases are ideal to store and manipulate scientific data. We refer to the official documentation for more information on SQLite.

Using the LJSQLITE3 library data can be conveniently exchanged between Lua and SQLite databases. Moreover arbitrary operations can be performed on SQLite databases directly from Lua:


local sql = require "ljsqlite3"
local conn = sql.open("") -- Open a temporary in-memory database.
 
-- Execute SQL commands separated by the ';' character:
conn:exec[[
CREATE TABLE t(id TEXT, num REAL);
INSERT INTO t VALUES('myid1', 200);
]]
 
-- Prepared statements are supported:
local stmt = conn:prepare("INSERT INTO t VALUES(?, ?)")
for i=2,4 do
  stmt:reset():bind('myid'..i, 200*i):step()
end
 
-- Command-line shell feature which here prints all records:
conn "SELECT * FROM t"
--> id    num
--> myid1 200
--> myid2 400
--> myid3 600
--> myid4 800
 
local t = conn:exec("SELECT * FROM t") -- Records are by column.
-- Access to columns via column numbers or names:
assert(t[1] == t.id)
-- Nested indexing corresponds to the record number:
assert(t[1][3] == 'myid3')
 
-- Convenience function returns multiple values for one record:
local id, num = conn:rowexec("SELECT * FROM t WHERE id=='myid3'")
print(id, num) --> myid3 600

-- Custom scalar function definition, aggregates supported as well.
conn:setscalar("MYFUN", function(x) return x/100 end)
conn "SELECT MYFUN(num) FROM t"
--> MYFUN(num)
--> 2
--> 4
--> 6
--> 8

conn:close() -- Close stmt as well.

SQLite Type Mappings

The SQLite datatypes page is a recommended reading. Summarizing, each element can only be of SQLite type: NULL, REAL, TEXT, BLOB or INTEGER. The declared type of each table column at the moment a table is created only represents a preferred type and any element in any column is allowed to be of any of the 5 types above. Here we focus only on the conversions between Lua types and SQLite types.

In the case of insertions via (Lua string) commands nothing is required on Lua side: the input SQLite types are determined by parsing the command (for instance 1.53 would be a REAL, 1 an INTEGER, '1.53' a TEXT and CAST('1.53' AS BLOB) a BLOB).

However any of the binding methods involve a mapping from Lua types to SQLite types and the resulting SQLite type is used as input for the insertion. Moreover records returned from SELECT queries necessarily involve a mapping from SQLite types to Lua types. The following two tables list the mappings in both directions where a luablob is obtained by calling sql.blob():

SQLite types → Lua types
InputOutputConversion
NULLnil
REALnumeric
TEXTstring[UTF-16 → UTF-8]
BLOBstring
INTEGERcdata<int64_t>
Lua types → SQLite types
InputOutputConversion
nilNULL
numericREAL
stringTEXT[UTF-8 → UTF-16]
luablobBLOB
cdata<int64_t>INTEGER

An empty conversion cell means that the data is mapped without modifications. For instance both Lua's numeric type and SQLite's REAL type refer to data represented by a floating-point double. The mapping between SQLite TEXT and Lua string involves a conversion between UTF-8 and UTF-16 text encodings in case the SQLite database has been configured to use UTF-16 encoding for TEXT storage. It's suggested to use databases configured as UTF-8 (the default) to avoid costly conversions.

Remember that a Lua string can store arbitrary data (plain text or binary data). In order to differentiate between SQLite TEXT and BLOB, where in the second case the content of the Lua string is always stored without text encoding modifications, the sql.blob() function can be called on the Lua string to return an object which forces the binding methods to threat the content of the Lua string as BLOB.

If the INTEGER → cdata<int64_t> mapping is not desired the following solutions can be considered:

API

In the following conn is an instance of a connection object which represents one or multiple (see the ATTACH DATABASE command) connections to SQLite databases and stmt is an instance of a prepared statement: an object associated to a given conn which represents an SQLite command which has been prepared for later execution. It is mainly used to improve the efficiency of SQLite operations as the preparation phase can be as expensive as the execution phase and prepared statements can be reused (see the stmt:reset() method). Moreover stmt supports the binding methods which allow to more efficient insertion of Lua data in databases by skipping the conversion of such data to strings. Any SQLite command is represented by a Lua string. All stmts which have been prepared by a conn always keep the conn alive as long as necessary.

sql = require "ljsqlite3"

Returns the loaded module (no global variable is set).

conn = sql.open(filename, mode = "rwc")

Opens a connection to the database specified by filename and returns a conn representing a connection to it. If filename is "" then a temporary database is created in memory which is destroyed once conn is closed (or collected). The optional argument mode can be one the following:

luablob = sql.blob(str)

Creates a luablob from its argument which must be of string type. It's only used in binding methods to distinguish between a string which represents text and one which represents binary data. The luablob wrapper makes sure that str is classified as an SQLite BLOB when passed to binding methods. A luablob object does not have any method associated with it and any operation on it (indexing, ...) is undefined. Example:


local conn = sql.open("") 
conn:exec("CREATE TABLE t(x)")
local stmt = conn:prepare("INSERT INTO t VALUES(?)")
stmt:reset():bind("atext"):step()
stmt:reset():bind(sql.blob("ablob")):step()
conn "SELECT x, typeof(x) FROM t"
--> x       typeof(x)
--> atext   text
--> ablob   blob
conn:close()

Closes all the database connections associated with conn, closes all the stmts associated with conn and un-registers all the scalar and aggregate custom functions associated with conn. It is an error to close conn more than once or to attempt any operation on a closed conn (the same holds for stmts). Any conn is automatically closed if necessary when garbage collected.

resultset, nrow = conn:exec(commands, get = "hik")

Executes SQLite commands separated by the ';' character (a trailing ';' is fine and is ignored). For all the commands except the last one the method ignore any resulting record. If the last command results in an empty result set, i.e. it is not a SELECT query or it is a SELECT query which produces 0 records, then nil, 0 is returned. Otherwise nrow is the number of records and resultset is a table whose structure depends on the value of the parameter get:


--[[ Suppose that the query returns data with the following layout:
       |    colname[   1]   |   ...  |   colname[ncol]
---------------------------------------------------------
     1 | record[   1][   1] |   ...  | record[   1][ncol]
     2 | record[   2][   1] |   ...  | record[   2][ncol]
   ... |        ...         |   ...  |        ...      
  nrow | record[nrow][   1] |   ...  | record[nrow][ncol]
]]
-- Then if get contains the character "h":
resultset[   0] = { colname[   1], ..., colname[ncol] }
-- If get contains the character "i":
resultset[   1] = { record[   1][   1], ..., record[ncol][   1] }
...
resultset[nrow] = { record[   1][nrow], ..., record[ncol][nrow] }
-- If get contains the character "k":
resultset[colname[   1]] = resultset[   1]
...
resultset[colname[ncol]] = resultset[ncol]

That is, the records are returned by column and are indexable using both the column numerical index and the column name. The method returns nrow as well because in case of NULL elements (which correspond to Lua nils) it would not be possible to determine the number of records resulting from the query. Examples of use:


local conn = sql.open("")
-- Returns nil:
conn:exec[[
CREATE TABLE t(id TEXT, num REAL);
INSERT INTO t VALUES('myid1', 200);
]]
-- Returns the resultset:
local t = conn:exec("SELECT * FROM t")
print(unpack(t[0]))      --> id    num
print(t.id[1], t.num[1]) --> myid1 200
print(t[1][1], t[2][1])  --> myid1 200
-- Probably a user mistake, returns nil:
local t = conn:exec[[
SELECT * FROM t;
INSERT INTO t VALUES('myid2', 400);
]]
assert(t == nil)
x_1, ..., x_ncol = conn:rowexec(command)

Executes a single command which must return at most one record. If the query results in no records nil is returned. Otherwise ncol values are returned which corresponds to the columns of the single record. Example:


local conn = sql.open("")
conn:exec[[
CREATE TABLE t(id TEXT, num REAL);
INSERT INTO t VALUES('myid1', 200);
]]
local id, num = conn:rowexec("SELECT * FROM t WHERE id=='myid1'")
print(id, num) --> myid1 200
conn(commands, out = print)

Executes SQLite commands separated by the ';' character (a trailing ';' is fine and is ignored) and passes all the records returned by all the commands preceded by the corresponding column names as strings to out (thus multiple parameters are passed to out() for each call). As out defaults to print this method mimics the Command Line Shell by default. Example:


local conn = sql.open("")
conn[[
CREATE TABLE t(id TEXT, num REAL);
INSERT INTO t VALUES('myid1', 200);
INSERT INTO t VALUES('myid2', 400);
SELECT * FROM t WHERE id=='myid1';
SELECT * FROM t WHERE id=='myid2';
]]
--> id    num
--> myid1 200
--> id    num
--> myid2 400
conn:setscalar(name, scalar); conn:setaggregate(name, initstate, step, final)

These methods register scalar and aggregate functions to a specific connection only with the specified name. It is not an error to register both a scalar and an aggregate with the same name. The argument name should be a string and the remaining arguments callable entities (functions or tables/cdatas with the __call() metamethod defined) with the following signatures:


-- Called for each record where ... = record column elements:
scalar    = function(...)        --> return mapping-allowed result
-- Called to initialise the aggregate computation:
initstate = function()           --> return state
-- Called for each record where ... = record column elements;
-- state is the persistent state of the computation:
step      = function(state, ...) --> return nothing
-- Called after all records has been passed to step;
-- sate is the persistent state of the computation:
final     = function(state)      --> return mapping-allowed result

By mapping-allowed we mean that the return type must be one of the Lua types listed in the input column of table Lua types → SQLite types. For the aggregate case the state can be any Lua object, it's created by calling initstate() and is passed back for each record as first argument to step() and after that as the unique argument to final() which returns the result of the computation. There is no state for scalar functions as it is not needed. To clear a registered function (and free the callback resource) simply call the methods conn:setscalar() and conn:setaggregate() passing name as the only argument. Example:


local conn = sql.open("")
conn:exec "CREATE TABLE t(num REAL);"
local stmt = conn:prepare("INSERT INTO t VALUES(?)")
for i=1,4 do stmt:reset():bind(i):step() end
conn:setscalar("MYSQRT", math.sqrt)
conn "SELECT MYSQRT(num) FROM t"
--> MYSQRT(num)
--> 1
--> 1.4142135623731
--> 1.7320508075689
--> 2
conn:setscalar("MYSQRT") -- Free callback.
conn:setaggregate("MYSUM",
  function() return { sum = 0 } end,
  function(self, x) self.sum = self.sum + x end,
  function(self) return self.sum end
)
conn "SELECT MYSUM(num) FROM t"
--> MYSUM(num)
--> 10
conn:setaggregate("MYSUM") -- Free callback.
stmt = conn:prepare(command)

Creates a stmt associated to conn from a single SQLite command.

stmt:close()

Closes (finalizes) a stmt. It is an error to close a stmt more than once or to attempt any operation on a closed stmt. Any stmt is closed automatically if necessary when garbage collected.

self = stmt:reset()

Resets a stmt to its initial state (i.e. just after conn:prepare()) so that it's ready to be used again. Resetting a stmt that is already in its initial state is allowed and has no effect. Returns the stmt itself allowing chaining, see example in next definition below.

row, colnames = stmt:step(row = {}, colnames = nil)

If stmt is in its initial state (i.e. just returned from conn:prepare() or stmt:reset() has just been called) the command from which stmt has been prepared gets executed. The argument row defaults to an empty table. If no record results from the executed command nil is returned. Otherwise row is used to store the record elements indexed by their numeric index and returned as first return value. Moreover if colnames is not nil then colnames is used to store the column names indexed by their numeric index and it is returned as second return value (otherwise only one value is returned). Further calls to step will return all the records in the order they are returned by the SQLite command according to the rules defined in the previous paragraph. When there are no records left step will return nil and so will do further calls to step:


local conn = sql.open("")
conn:exec[[
CREATE TABLE t(id TEXT, num REAL);
INSERT INTO t VALUES('myid1', 200);
INSERT INTO t VALUES('myid2', 400);
]]
local stmt = conn:prepare("SELECT * FROM t")
local row, names = stmt:step({}, {})
print(unpack(names))
print(unpack(row))
while stmt:step(row) do
  print(unpack(row))
end
--> id    num
--> myid1 200
--> myid2 400
resultset, nrecords = stmt:resultset(get = "hik", maxrecords = nil)

This method behaves like conn:exec(): it returns the remaining records resulting from the command from which stmt has been prepared according to the same rules of conn:exec() up to a maximum of maxrecords if maxrecords is not nil (otherwise all the remaining records are returned). Remark: for both stmt:step() and stmt:resultset() methods it may be useful to think of stmt as a file descriptor: we traverse the records (lines) from the first to the last:


local stmt = conn:prepare("SELECT * FROM some_table")
while true do
  -- Retrieve the records in chunks of maximum 1000 records.
  local partial, nr = stmt:resultset(1000)
    -- Do something with them ...
  if not partial then break end -- Stop when no records left.
end
self = stmt:bind1(i, value)

This method has to be used with statements which have been prepared for later binding according to SQLite parameters syntax. Please notice that only parameters indexed by a numerical value are supported. stmt:bind1() binds a single mapping-allowed Lua value (i.e. in the input column of table Lua types → SQLite types) to the i-th parameter of the query. It is an error for i to be out of the allowed bounds. Any parameter which has not been binded is set to NULL. Binded parameters persist among multiple calls to stmt:reset() and stmt:step(): stmt:clearbind() must be used to set all the parameters back to NULL). Example:


local conn = sql.open("")
conn:exec "CREATE TABLE t(r REAL, i INTEGER, s TEXT, b BLOB);"
local stmt = conn:prepare "INSERT INTO t VALUES(?, ?, ?, ?)"
stmt:reset():bind(1, 1, "astr", "astr"):step()
stmt:reset():bind(2, 2, "bstr", sql.blob("bblob")):step()
stmt:reset():step()
stmt:reset():bind1(2, 4LL):step()
stmt:reset():clearbind():bind1(2, 5LL):step()
conn "SELECT * FROM t"
--> r       i       s       b
--> 1       1LL     astr    astr
--> 2       2LL     bstr    bblob
--> 2       2LL     bstr    bblob
--> 2       4LL     bstr    bblob
-->         5LL
self = stmt:bind(...)

This method is equivalent to calling stmt:bind1(i, v) once for every passed argument where i is the position of the argument and v the argument itself. See example in stmt:bind1().

self = stmt:clearbind()

Sets back all the parameters to NULL. See example in stmt:bind1().