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<title>SQLite Frequently Asked Questions</title>

set cnt 1
proc faq {question answer} {
  set ::faq($::cnt) [list [string trim $question] [string trim $answer]]
  incr ::cnt

# Enter questions and answers here.

faq {
  How do I create an AUTOINCREMENT field.
} {
  <p>Short answer: A column declared [INTEGER PRIMARY KEY] will

  <p>Longer answer:
  If you declare a column of a table to be [INTEGER PRIMARY KEY], then
  whenever you insert a NULL
  into that column of the table, the NULL is automatically converted
  into an integer which is one greater than the largest value of that
  column over all other rows in the table, or 1 if the table is empty.
  Or, if the largest existing integer key 9223372036854775807 is in use then an
  unused key value is chosen at random.
  For example, suppose you have a table like this:
  <p>With this table, the statement</p>
  <p>is logically equivalent to saying:</p>
INSERT INTO t1 VALUES((SELECT max(a) FROM t1)+1,123);

  <p>There is a function named
  [sqlite3_last_insert_rowid()] which will return the integer key
  for the most recent insert operation.</p>

  <p>Note that the integer key is one greater than the largest
  key that was in the table just prior to the insert.  The new key
  will be unique over all keys currently in the table, but it might
  overlap with keys that have been previously deleted from the
  table.  To create keys that are unique over the lifetime of the
  table, add the [AUTOINCREMENT] keyword to the [INTEGER PRIMARY KEY]
  declaration.  Then the key chosen will be one more than the
  largest key that has ever existed in that table.  If the largest
  possible key has previously existed in that table, then the [INSERT]
  will fail with an [SQLITE_FULL] error code.</p>

faq {
  What datatypes does SQLite support?
} {
  SQLite uses [dynamic typing].  Content can be stored as INTEGER,

faq {
  SQLite lets me insert a string into a database column of type integer!
} {
  <p>This is a feature, not a bug.  SQLite uses [dynamic typing]. 
  It does not enforce data type  constraints.  Data of any type can
  (usually) be inserted into any column.  You can put arbitrary length
  strings into integer columns, floating point numbers in boolean columns,
  or dates in character columns.  The [datatype] you assign to a column in the
  CREATE TABLE command does not restrict what data can be put into
  that column.  Every column is able to hold
  an arbitrary length string.  (There is one exception: Columns of
  type [INTEGER PRIMARY KEY] may only hold a 64-bit signed integer.
  An error will result
  if you try to put anything other than an integer into an
  [INTEGER PRIMARY KEY] column.)</p>

  <p>But SQLite does use the declared type of a column as a hint
  that you prefer values in that format.  So, for example, if a
  column is of type INTEGER and you try to insert a string into
  that column, SQLite will attempt to convert the string into an
  integer.  If it can, it inserts the integer instead.  If not,
  it inserts the string.  This feature is called [affinity | type affinity].

faq {
  Why doesn't SQLite allow me to use '0' and '0.0' as the primary
  key on two different rows of the same table?
} {
  <p>This problem occurs when your primary key is a numeric type.  Change the
  [datatype] of your primary key to TEXT and it should work.</p>

  <p>Every row must have a unique primary key.  For a column with a
  numeric type, SQLite thinks that <b>'0'</b> and <b>'0.0'</b> are the
  same value because they compare equal to one another numerically.
  (See the previous question.)  Hence the values are not unique.</p>

faq {
  Can multiple applications or multiple instances of the same
  application access a single database file at the same time?
} {
  <p>Multiple processes can have the same database open at the same
  time.  Multiple processes can be doing a SELECT
  at the same time.  But only one process can be making changes to
  the database at any moment in time, however.</p>

  <p>SQLite uses reader/writer locks to control access to the database.
  (Under Win95/98/ME which lacks support for reader/writer locks, a
  probabilistic simulation is used instead.)
  But use caution: this locking mechanism might
  not work correctly if the database file is kept on an NFS filesystem.
  This is because fcntl() file locking is broken on many NFS implementations.
  You should avoid putting SQLite database files on NFS if multiple
  processes might try to access the file at the same time.  On Windows,
  Microsoft's documentation says that locking may not work under FAT
  filesystems if you are not running the Share.exe daemon.  People who
  have a lot of experience with Windows tell me that file locking of
  network files is very buggy and is not dependable.  If what they
  say is true, sharing an SQLite database between two or more Windows
  machines might cause unexpected problems.</p>

  <p>We are aware of no other <i>embedded</i> SQL database engine that
  supports as much concurrency as SQLite.  SQLite allows multiple processes
  to have the database file open at once, and for multiple processes to
  read the database at once.  When any process wants to write, it must
  lock the entire database file for the duration of its update.  But that
  normally only takes a few milliseconds.  Other processes just wait on
  the writer to finish then continue about their business.  Other embedded
  SQL database engines typically only allow a single process to connect to
  the database at once.</p>

  <p>However, client/server database engines (such as PostgreSQL, MySQL,
  or Oracle) usually support a higher level of concurrency and allow
  multiple processes to be writing to the same database at the same time.
  This is possible in a client/server database because there is always a
  single well-controlled server process available to coordinate access.
  If your application has a need for a lot of concurrency, then you should
  consider using a client/server database.  But experience suggests that
  most applications need much less concurrency than their designers imagine.

  <p>When SQLite tries to access a file that is locked by another
  process, the default behavior is to return SQLITE_BUSY.  You can
  adjust this behavior from C code using the 
  [sqlite3_busy_handler()] or [sqlite3_busy_timeout()]
  API functions.</p>

faq {
  Is SQLite threadsafe?
} {
  <p>[ | Threads are evil].
  Avoid them.

  <p>SQLite is threadsafe.  We make this concession since many users choose
  to ignore the advice given in the previous paragraph.
  But in order to be thread-safe, SQLite must be compiled
  with the SQLITE_THREADSAFE preprocessor macro set to 1.  Both the Windows
  and Linux precompiled binaries in the distribution are compiled this way.
  If you are unsure if the SQLite library you are linking against is compiled
  to be threadsafe you can call the [sqlite3_threadsafe()]
  interface to find out.

  <p>SQLite is threadsafe because it uses mutexes to serialize
  access to common data structures.  However, the work of acquiring and
  releasing these mutexes will slow SQLite down slightly.  Hence, if you
  do not need SQLite to be threadsafe, you should disable the mutexes
  for maximum performance.  See the [threading mode] documentation for
  additional information.</p>

  <p>Under Unix, you should not carry an open SQLite database across
  a fork() system call into the child process.</p>

faq {
  How do I list all tables/indices contained in an SQLite database
} {
  <p>If you are running the <b>sqlite3</b> command-line access program
  you can type "<b>.tables</b>" to get a list of all tables.  Or you
  can type "<b>.schema</b>" to see the complete database schema including
  all tables and indices.  Either of these commands can be followed by
  a LIKE pattern that will restrict the tables that are displayed.</p>

  <p>From within a C/C++ program (or a script using Tcl/Ruby/Perl/Python
  bindings) you can get access to table and index names by doing a SELECT
  on a special table named "<b>SQLITE_MASTER</b>".  Every SQLite database
  has an SQLITE_MASTER table that defines the schema for the database.
  The SQLITE_MASTER table looks like this:</p>
CREATE TABLE sqlite_master (
  type TEXT,
  name TEXT,
  tbl_name TEXT,
  rootpage INTEGER,
  sql TEXT
  <p>For tables, the <b>type</b> field will always be <b>'table'</b> and the
  <b>name</b> field will be the name of the table.  So to get a list of
  all tables in the database, use the following SELECT command:</p>
SELECT name FROM sqlite_master
WHERE type='table'
ORDER BY name;
  <p>For indices, <b>type</b> is equal to <b>'index'</b>, <b>name</b> is the
  name of the index and <b>tbl_name</b> is the name of the table to which
  the index belongs.  For both tables and indices, the <b>sql</b> field is
  the text of the original CREATE TABLE or CREATE INDEX statement that
  created the table or index.  For automatically created indices (used
  to implement the PRIMARY KEY or UNIQUE constraints) the <b>sql</b> field
  is NULL.</p>

  <p>The SQLITE_MASTER table is read-only.  You cannot change this table
  using UPDATE, INSERT, or DELETE.  The table is automatically updated by

  <p>Temporary tables do not appear in the SQLITE_MASTER table.  Temporary
  tables and their indices and triggers occur in another special table
  except that it is only visible to the application that created the 
  temporary tables.  To get a list of all tables, both permanent and
  temporary, one can use a command similar to the following:
   (SELECT * FROM sqlite_master UNION ALL
    SELECT * FROM sqlite_temp_master)
WHERE type='table'

faq {
  Are there any known size limits to SQLite databases?
} {
  <p>See <a href="limits.html">limits.html</a> for a full discussion of
  the limits of SQLite.</p>

faq {
  What is the maximum size of a VARCHAR in SQLite?
} {
  <p>SQLite does not enforce the length of a VARCHAR.  You can declare
  a VARCHAR(10) and SQLite will be happy to store a 500-million character
  string there.  And it will keep all 500-million characters intact.
  Your content is never truncated.  SQLite understands the column type
  of "VARCHAR(<i>N</i>)" to be the same as "TEXT", regardless of the value
  of <i>N</i>.

faq {
  Does SQLite support a BLOB type?
} {
  <p>SQLite allows you to store BLOB data in any 
  column, even columns that are declared to hold some other type.
  BLOBs can even be used as PRIMARY KEYs.</p>

faq {
  How do I add or delete columns from an existing table in SQLite.
} {
  <p>SQLite has limited 
  <a href="lang_altertable.html">ALTER TABLE</a> support that you can
  use to add a column to the end of a table or to change the name of
  a table.  
  If you want to make more complex changes in the structure of a table,
  you will have to recreate the
  table.  You can save existing data to a temporary table, drop the
  old table, create the new table, then copy the data back in from
  the temporary table.</p>

  <p>For example, suppose you have a table named "t1" with columns
  names "a", "b", and "c" and that you want to delete column "c" from
  this table.  The following steps illustrate how this could be done:

INSERT INTO t1_backup SELECT a,b FROM t1;
INSERT INTO t1 SELECT a,b FROM t1_backup;
DROP TABLE t1_backup;

faq {
  I deleted a lot of data but the database file did not get any
  smaller.  Is this a bug?
} {
  <p>No.  When you delete information from an SQLite database, the
  unused disk space is added to an internal "free-list" and is reused
  the next time you insert data.  The disk space is not lost.  But
  neither is it returned to the operating system.</p>

  <p>If you delete a lot of data and want to shrink the database file,
  run the <a href="lang_vacuum.html">VACUUM</a> command.
  VACUUM will reconstruct
  the database from scratch.  This will leave the database with an empty
  free-list and a file that is minimal in size.  Note, however, that the
  VACUUM can take some time to run and it can use up to twice
  as much temporary disk space as the original file while it is running.

  <p>An alternative to using the VACUUM command
  is auto-vacuum mode, enabled using the 
  <a href="pragma.html#pragma_auto_vacuum">auto_vacuum pragma</a>.</p>

faq {
  Can I use SQLite in my commercial product without paying royalties?
} {
  <p>Yes.  SQLite is in the 
  <a href="copyright.html">public domain</a>.  No claim of ownership is made
  to any part of the code.  You can do anything you want with it.</p>

faq {
  How do I use a string literal that contains an embedded single-quote (')
} {
  <p>The SQL standard specifies that single-quotes in strings are escaped
  by putting two single quotes in a row.  SQL works like the Pascal programming
  language in this regard. Example:

    INSERT INTO xyz VALUES('5 O''clock');

faq {
  What is an SQLITE_SCHEMA error, and why am I getting one?
} {
  <p>An [SQLITE_SCHEMA] error is returned when a 
  prepared SQL statement is no longer valid and cannot be executed.
  When this occurs, the statement must be recompiled from SQL using 
  the [sqlite3_prepare()] API.
  An SQLITE_SCHEMA error can only occur when using the [sqlite3_prepare()],
  and [sqlite3_step()] interfaces to run SQL.
  You will never receive an [SQLITE_SCHEMA] error from
  [sqlite3_exec()].  Nor will you receive an error if you
  prepare statements using [sqlite3_prepare_v2()] instead of

  <p>The [sqlite3_prepare_v2()] interface creates a
  [prepared statement] that will automatically recompile itself if
  the schema changes.  The easiest way to deal with
  [SQLITE_SCHEMA] errors is to always use [sqlite3_prepare_v2()]
  instead of [sqlite3_prepare()].

faq {
  Why does ROUND(9.95,1)  return 9.9 instead of 10.0?
  Shouldn't 9.95 round up?
} {
  <p>SQLite uses binary arithmetic and in binary, there is no
  way to write 9.95 in a finite number of bits.  The closest to
  you can get to 9.95 in a 64-bit IEEE float (which is what
  SQLite uses) is 9.949999999999999289457264239899814128875732421875.
  So when you type "9.95", SQLite really understands the number to be
  the much longer value shown above.  And that value rounds down.</p>

  <p>This kind of problem comes up all the time when dealing with
  floating point binary numbers.  The general rule to remember is
  that most fractional numbers that have a finite representation in decimal
  (a.k.a "base-10")
  do not have a finite representation in binary (a.k.a "base-2").
  And so they are
  approximated using the closest binary number available.  That
  approximation is usually very close, but it will be slightly off
  and in some cases can cause your results to be a little different
  from what you might expect.</p>

faq {
  I get some compiler warnings when I compile SQLite.
  Isn't this a problem?  Doesn't it indicate poor code quality?
} {
  <p>Quality assurance in SQLite is done using 
  [test coverage | full-coverage testing],
  not by compiler warnings or other static code analysis tools.
  In other words, we verify that SQLite actually gets the
  correct answer, not that it merely satisfies stylistic constraints.
  Most of the SQLite code base is devoted purely to testing.
  The SQLite test suite runs tens of thousands of separate test cases and
  many of those test cases are parameterized so that hundreds of millions
  of tests involving billions of SQL statements are run and evaluated
  for correctness prior to every release.  The developers use code
  coverage tools to verify that all paths through the code are tested.
  Whenever a bug is found in SQLite, new test cases are written to
  exhibit the bug so that the bug cannot recur undetected in the future.</p>

  <p>During testing, the SQLite library is compiled with special
  instrumentation that allows the test scripts to simulate a wide
  variety of failures in order to verify that SQLite recovers
  correctly.  Memory allocation is carefully tracked and no memory
  leaks occur, even following memory allocation failures.  A custom
  VFS layer is used to simulate operating system crashes and power
  failures in order to ensure that transactions are atomic across
  these events.  A mechanism for deliberately injecting I/O errors
  shows that SQLite is resilient to such malfunctions.  (As an
  experiment, try inducing these kinds of errors on other SQL database
  engines and see what happens!)</p>

  <p>We also run SQLite using [ | Valgrind]
  on Linux and verify that it detects no problems.</p>

  <p>Some people say that we should eliminate all warnings because
  benign warnings mask real warnings that might arise in future changes.
  This is true enough.  But in reply, the developers observe that all
  warnings have already been fixed in the builds
  used for SQLite development (various versions of GCC, MSVC,
  and clang).
  Compiler warnings usually only arise from compilers or compile-time 
  options that the SQLite developers to not use themselves.</p>

faq {
  Case-insensitive matching of Unicode characters does not work.
} {
  The default configuration of SQLite only supports case-insensitive
  comparisons of ASCII characters.  The reason for this is that doing
  full Unicode case-insensitive comparisons and case conversions 
  requires tables and logic that would nearly double the size of
  the SQLite library.  The
  SQLite developers reason that any application that needs full
  Unicode case support probably already has the necessary tables and
  functions and so SQLite should not take up space to 
  duplicate this ability.</p>

  <p>Instead of providing full Unicode case support by default, 
  SQLite provides the ability to link against external
  Unicode comparison and conversion routines.
  The application can overload the built-in [NOCASE] collating
  sequence (using [sqlite3_create_collation()]) and the built-in
  [like()], [upper()], and [lower()] functions
  (using [sqlite3_create_function()]).  
  The SQLite source code includes an "ICU" extension that does 
  these overloads.  Or, developers can write their own overloads
  based on their own Unicode-aware comparison routines already
  contained within their project.

faq {
  INSERT is really slow - I can only do few dozen INSERTs per second
} {
  Actually, SQLite will easily do 50,000 or more [INSERT] statements per second
  on an average desktop computer.  But it will only do a few dozen transactions
  per second.  Transaction speed is limited by the rotational speed of
  your disk drive.  A transaction normally requires two complete rotations
  of the disk platter, which on a 7200RPM disk drive limits you to about
  60 transactions per second.

  <p>Transaction speed is limited by disk drive speed because (by default)
  SQLite actually waits until the data really is safely stored on the disk
  surface before the transaction is complete.  That way, if you suddenly lose
  power or if your OS crashes, your data is still safe.  For details, 
  read about <a href="atomiccommit.html">atomic commit in SQLite.</a>.

  <p>By default, each INSERT statement is its own transaction.  But if you
  surround multiple INSERT statements with [BEGIN]...[COMMIT] then all the
  inserts are grouped into a single transaction.  The time needed to commit
  the transaction is amortized over all the enclosed insert statements and
  so the time per insert statement is greatly reduced.

  <p>Another option is to run [PRAGMA synchronous=OFF].  This command will
  cause SQLite to not wait on data to reach the disk surface, which will make
  write operations appear to be much faster.  But if you lose power in the
  middle of a transaction, your database file might go corrupt.

faq {
  I accidentally deleted some important information from my SQLite database.
  How can I recover it?
} {
  If you have a backup copy of your database file, recover the information
  from your backup.

  <p>If you do not have a backup, recovery is very difficult.  You might
  be able to find partial string data in a binary dump of the raw database
  file.  Recovering numeric data might also be possible given special tools,
  though to our knowledge no such tools exist.  SQLite is sometimes compiled
  with the [SQLITE_SECURE_DELETE] option which overwrites all deleted content
  with zeros.  If that is the case then recovery is clearly impossible.
  Recovery is also impossible if you have run [VACUUM] since the data was
  deleted.  If SQLITE_SECURE_DELETE is not used and VACUUM has not been run,
  then some of the deleted content might still be in the database file, in
  areas marked for reuse.  But, again, there exist no procedures or tools
  that we know of to help you recover that data.

faq {
  What is an SQLITE_CORRUPT error?  What does it mean for the database
  to be "malformed"? Why am I getting this error?
} {
  <p>An [SQLITE_CORRUPT] error is returned when SQLite detects an error
  in the structure, format, or other control elements of the
  database file.</p>

  <p>SQLite does not corrupt database files, except in the case of very
  rare bugs (see 
  <a href="">DatabaseCorruption</a>) 
  and even then the bugs are normally difficult to
  reproduce.  Even if your application crashes in the middle of an
  update, your database is safe.  The database is safe even if your OS
  crashes or takes a power loss.  The crash-resistance of SQLite has
  been extensively studied and tested and is attested by years of real-world 
  experience by billions of users.</p>

  <p>That said, there are a number of things that external programs or bugs
  in your hardware or OS can do to corrupt a database file.  See
  <a href="howtocorrupt.html">How To Corrupt An SQLite DAtabase File</a> for
  further information.

  <p>Your can use <a href="pragma.html#pragma_integrity_check">PRAGMA integrity_check</a> 
  to do a thorough but time intensive test of the database integrity.</p>

  <p>Your can use <a href="pragma.html#pragma_quick_check">PRAGMA quick_check</a> to do a faster 
  but less thorough test of the database integrity.</p>

  <p>Depending how badly your database is corrupted, you may be able to 
  recover some of the data by using the CLI to dump the schema and contents
  to a file and then recreate.  Unfortunately, once humpty-dumpty falls off 
  the wall, it is generally not possible to put him back together again.</p>

faq {
  Does SQLite support foreign keys?
} {
  As of version 3.6.19, SQLite supports [foreign key constraints].  But enforcement
  of foreign key constraints is turned off by default (for backwards compatibility).
  To enable foreign key constraint enforcements, run 
  [PRAGMA foreign_keys|PRAGMA foreign_keys=ON] or compile with

faq {
  I get a compiler error if I use the SQLITE_OMIT_... 
  compile-time options when building SQLite.
} {
  The [omitfeatures | SQLITE_OMIT_...] compile-time options only work
  when building from canonical source files.  They do <u>not</u> work
  when you build from the SQLite [amalgamation] or from the pre-processed
  source files.

  <p>It is possible to build a special [amalgamation] that will work with
  a predetermined set of SQLITE_OMIT_... options.  Instructions for doing
  so can be found with the [omitfeatures | SQLITE_OMIT_... documentation].

faq {
  My WHERE clause expression <tt>column1="column1"</tt> does not work.
  It causes every row of the table to be returned, not just the rows
  where column1 has the value "column1".
} {
  Use single-quotes, not double-quotes, around string literals in SQL.
  This is what the SQL standard requires.  Your WHERE clause expression
  should read: <tt>column1='column1'</tt>

  <p>SQL uses double-quotes around identifiers (column or table names) that
  contains special characters or which are keywords.  So double-quotes are
  a way of escaping identifier names.  Hence, when you say
  <tt>column1="column1"</tt> that is equivalent to 
  <tt>column1=column1</tt> which is obviously always true.

faq {
  How are the syntax diagrams (a.k.a. "railroad" diagrams) for
  SQLite generated?
} {
  The process is explained at [].

faq {
  The SQL standard requires that a UNIQUE constraint be enforced even if
  one or more of the columns in the constraint are NULL, but SQLite does
  not do this.  Isn't that a bug?
} {
  Perhaps you are referring to the following statement from SQL92:

  A unique constraint is satisfied if and only if no two rows in a
  table have the same non-null values in the unique columns. 

  That statement is ambiguous, having at least two possible interpretations:

  <li>A unique constraint is satisfied if and only if no two rows in a
  table have the same values and have non-null values in the unique columns.
  <li>A unique constraint is satisfied if and only if no two rows in a 
  table have the same values in the subset of unique columns that are not null.

  SQLite follows interpretation (1), as does PostgreSQL, MySQL, Oracle,
  and Firebird.  It is true that Informix and Microsoft SQL Server use
  interpretation (2), however we the SQLite developers hold that 
  interpretation (1) is the most natural reading 
  of the requirement and we also want to maximize compatibility with other
  SQL database engines, and most other database engines also go with (1),
  so that is what SQLite does.

faq {
  What is the Export Control Classification Number (ECCN) for SQLite?
} {
  After careful review of the Commerce Control List (CCL), we are convinced
  that the core public-domain SQLite source code is not described by any ECCN,
  hence the ECCN should be reported as <b>EAR99</b>.

  <p>The above is true for the core public-domain SQLite.  If you extend
  SQLite by adding new code, or if you statically link SQLite with your
  application, that might change the ECCN in your particular case.

faq {
  My query does not return the column name that I expect.  Is this a bug?
} {
  If the columns of your result set are named by AS clauses, then SQLite
  is guaranteed to use the identifer to the right of the AS keyword as the
  column name.  If the result set does not use an AS clause, then SQLite
  is free to name the column anything it wants.
  See the [sqlite3_column_name()] documentation for further information.

# End of questions and answers.

hd_puts {<h2>Frequently Asked Questions</h2>}

    hd_puts {<oL>}
for {set i 1} {$i<$cnt} {incr i} {
  hd_puts "<li><a href=\"#q$i\">[lindex $faq($i) 0]</a></li>"
hd_puts {</ol>}

for {set i 1} {$i<$cnt} {incr i} {
  hd_fragment q$i
  hd_puts "<p><b>($i) [lindex $faq($i) 0]</b></p>\n"
  hd_resolve "<blockquote>[lindex $faq($i) 1]</blockquote></li>\n"
hd_puts {</ol>}