This article provides an overview and roadmap to the C/C++ interface to SQLite.
Early versions of SQLite were very easy to learn since they only supported 5 C/C++ interfaces. But as SQLite has grown in capability, new C/C++ interfaces have been added so that now there are over 185 distinct APIs. This can be overwhelming to a new programmer. Fortunately, most of the C/C++ interfaces in SQLite are very specialized and never need to be used. Despite having so many entry points, the core API is still relatively simple and easy to code to. This article aims to provide all of the background information needed to easily understand how SQLite works.
A separate document, [capi3ref | The SQLite C/C++ Interface], provides detailed specifications for all of the various C/C++ APIs for SQLite. Once the reader understands the basic principles of operation for SQLite, [capi3ref | that document] should be used as a reference guide. This article is intended as introduction only and is neither a complete nor authoritative reference for the SQLite API.
The principal task of an SQL database engine is to evaluate statements of SQL. In order to accomplish this purpose, the developer needs to know about two objects:
Strictly speaking, the [prepared statement] object is not required since the convenience wrapper interfaces, [sqlite3_exec] or [sqlite3_get_table], can be used and these convenience wrappers encapsulate and hide the [prepared statement] object. Nevertheless, an understanding of [prepared statements] is needed to make full use of SQLite.
The [database connection] and [prepared statement] objects are controlled by a small set of C/C++ interface routine listed below.
The six C/C++ interface routines and two objects listed above form the core functionality of SQLite. The developer who understands them will have a good foundation for using SQLite.
Note that the list of routines is conceptual rather than actual. Many of these routines come in multiple versions. For example, the list above shows a single routine named [sqlite3_open()] when in fact there are three separate routines that accomplish the same thing in slightly different ways: [sqlite3_open()], [sqlite3_open16()] and [sqlite3_open_v2()]. The list mentions [sqlite3_column_int | sqlite3_column()] when in fact no such routine exists. The "sqlite3_column()" shown in the list is place holders for an entire family of routines to be used for extracting column data in various datatypes.
Here is a summary of what the core interfaces do:
|[sqlite3_open()]||This routine opens a connection to an SQLite database file and returns a [database connection] object. This is often the first SQLite API call that an application makes and is a prerequisite for most other SQLite APIs. Many SQLite interfaces require a pointer to the [database connection] object as their first parameter and can be thought of as methods on the [database connection] object. This routine is the constructor for the [database connection] object.|
converts SQL text into a [prepared statement] object and returns a pointer
to that object. This interface requires a [database connection] pointer
created by a prior call to [sqlite3_open()] and a text string containing
the SQL statement to be prepared. This API does not actually evaluate
the SQL statement. It merely prepares the SQL statement for evaluation.
Note that the use of [sqlite3_prepare()] is not recommended for new applications. The alternative routine [sqlite3_prepare_v2()] should be used instead.
|[sqlite3_step()]||This routine is used to evaluate a [prepared statement] that has been previously created by the [sqlite3_prepare()] interface. The statement is evaluated up to the point where the first row of results are available. To advance to the second row of results, invoke [sqlite3_step()] again. Continue invoking [sqlite3_step()] until the statement is complete. Statements that do not return results (ex: INSERT, UPDATE, or DELETE statements) run to completion on a single call to [sqlite3_step()].|
|[sqlite3_column_int | sqlite3_column()]||
This routine returns a single column from the current row of a result
set for a [prepared statement] that is being evaluated by [sqlite3_step()].
Each time [sqlite3_step()] stops with a new result set row, this routine
can be called multiple times to find the values of all columns in that row.
As noted above, there really is no such thing as a "sqlite3_column()"
function in the SQLite API. Instead, what we here call "sqlite3_column()"
is really a place-holder for an entire family of functions that return
a value from the result set in various data types. There are also routines
in this family that return the size of the result (if it is a string or
BLOB) and the number of columns in the result set.
|[sqlite3_finalize()]||This routine destroys a [prepared statement] created by a prior call to [sqlite3_prepare()]. Every prepared statement must be destroyed using a call to this routine in order to avoid memory leaks.|
|[sqlite3_close()]||This routine closes a [database connection] previously opened by a call to [sqlite3_open()]. All [prepared statements] associated with the connection should be [sqlite3_finalize | finalized] prior to closing the connection.|
An application that wants to use SQLite will typically use [sqlite3_open()] to create a single [database connection] during initialization. Note that [sqlite3_open()] can be used to either open existing database files or to create and open new database files. While many applications use only a single [database connection], there is no reason why an application cannot call [sqlite3_open()] multiple times in order to open multiple [database connections] - either to the same database or to different databases. Sometimes a multi-threaded application will create separate [database connections] for each threads. Note too that is not necessary to open separate database connections in order to access two or more databases. A single [database connection] can be made to access two or more databases at one time using the [ATTACH] SQL command.
Many applications destroy their [database connections] using calls to [sqlite3_close()] at shutdown. Or, for example, an application might open [database connections] in response to a File->Open menu action and then destroy the corresponding [database connection] in response to the File->Close menu.
To run an SQL statement, the application follows these steps:
The foregoing is all one really needs to know in order to use SQLite effectively. All the rest is just ornamentation and detail.
The [sqlite3_exec()] interface is a convenience wrapper that carries out all four of the above steps with a single function call. A callback function passed into [sqlite3_exec()] is used to process each row of the result set. The [sqlite3_get_table()] is another convenience wrapper that does all four of the above steps. The [sqlite3_get_table()] interface differs from [sqlite3_exec()] in that it stores the results of queries in heap memory rather than invoking a callback.
It is important to realize that neither [sqlite3_exec()] nor [sqlite3_get_table()] do anything that cannot be accomplished using the core routines. In fact, these wrappers are implemented purely in terms of the core routines.
In prior discussion, it was assumed that each SQL statement is prepared once, evaluated, then destroyed. However, the SQLite allows the same [prepared statement] to be evaluated multiple times. This is accomplished using the following routines:
After a [prepared statement] has been evaluated by one or more calls to [sqlite3_step()], it can be reset in order to be evaluated again by a call to [sqlite3_reset()]. Using [sqlite3_reset()] on an existing [prepared statement] rather than creating a new [prepared statement] avoids unnecessary calls to [sqlite3_prepare()]. In many SQL statements, the time needed to run [sqlite3_prepare()] equals or exceeds the time needed by [sqlite3_step()]. So avoiding calls to [sqlite3_prepare()] can result in a significant performance improvement.
Usually, though, it is not useful to evaluate exactly the same SQL statement more than once. More often, one wants to evaluate similar statements. For example, you might want to evaluate an INSERT statement multiple times though with different values to insert. To accommodate this kind of flexibility, SQLite allows SQL statements to contain [parameter | parameters] which are "bound" to values prior to being evaluated. These values can later be changed and the same [prepared statement] can be evaluated a second time using the new values.
In SQLite, wherever it is valid to include a string literal, one can use a [parameter] in one of the following forms:
In the examples above, NNN is an integer value and AAA is an identifier. A parameter initially has a value of NULL. Prior to calling [sqlite3_step()] for the first time or immediately after [sqlite3_reset()], the application can invoke one of the [sqlite3_bind_int | sqlite3_bind()] interfaces to attach values to the parameters. Each call to [sqlite3_bind_int | sqlite3_bind()] overrides prior bindings on the same parameter.
An application is allowed to prepare multiple SQL statements in advance and evaluate them as needed. There is no arbitrary limit to the number of outstanding [prepared statements].
SQLite includes interfaces that can be used to extend its functionality. Such routines include:
The [sqlite3_create_collation()] interface is used to create new collating sequences for sorting text. The [sqlite3_create_module()] interface is used to register new virtual table implementations.
The [sqlite3_create_function()] interface creates new SQL functions - either scalar or aggregate. The new function implementation typically makes use of the following additional interfaces:
All of the built-in SQL functions of SQLite are created using exactly these same interfaces. Refer to the SQLite source code, and in particular the date.c and func.c source files for examples.
This article only mentions the foundational SQLite interfaces. The SQLite library includes many other APIs implementing useful features that are not described here. A [capi3ref_funclist | complete list of functions] that form the SQLite application programming interface is found at the [capi3ref | C/C++ Interface Specification]. Refer to that document for complete and authoritative information about all SQLite interfaces.