Reading Column Data Instead of Rows Sqlite

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Frequently Asked Questions

1. How do I create an AUTOINCREMENT field?
2. What datatypes does SQLite support?
3. SQLite lets me insert a string into a database column of blazon integer!
4. Why doesn't SQLite allow me to use '0' and '0.0' as the master key on two different rows of the same table?
5. Tin multiple applications or multiple instances of the same application access a single database file at the same time?
6. Is SQLite threadsafe?
7. How do I list all tables/indices contained in an SQLite database
8. Are at that place any known size limits to SQLite databases?
9. What is the maximum size of a VARCHAR in SQLite?
10. Does SQLite support a BLOB type?
11. How do I add together, delete or rename columns from an existing table in SQLite?
12. I deleted a lot of data just the database file did not get whatsoever smaller. Is this a bug?
13. Tin I use SQLite in my commercial product without paying royalties?
14. How practice I apply a string literal that contains an embedded single-quote (') graphic symbol?
15. What is an SQLITE_SCHEMA error, and why am I getting one?
17. I get some compiler warnings when I compile SQLite. Isn't this a trouble? Doesn't it betoken poor code quality?
18. Case-insensitive matching of Unicode characters does not piece of work.
19. INSERT is really slow - I can but exercise few dozen INSERTs per second
20. I accidentally deleted some important data from my SQLite database. How can I recover it?
21. What is an SQLITE_CORRUPT error? What does it mean for the database to be "malformed"? Why am I getting this error?
22. Does SQLite support foreign keys?
23. I go a compiler error if I utilize the SQLITE_OMIT_... compile-fourth dimension options when edifice SQLite.
24. My WHERE clause expression column1="column1" does not work. It causes every row of the table to be returned, not merely the rows where column1 has the value "column1".
25. How are the syntax diagrams (a.grand.a. "railroad" diagrams) for SQLite generated?
26. The SQL standard requires that a UNIQUE constraint be enforced even if one or more of the columns in the constraint are NULL, merely SQLite does non exercise this. Isn't that a problems?
27. What is the Export Control Classification Number (ECCN) for SQLite?
28. My query does not return the column name that I wait. Is this a bug?

(1) How exercise I create an AUTOINCREMENT field?

Curt answer: A column alleged INTEGER PRIMARY KEY will autoincrement.

Longer answer: If yous declare a column of a tabular array to be INTEGER Main KEY, and so whenever you insert a NULL into that cavalcade of the table, the Cypher is automatically converted into an integer which is 1 greater than the largest value of that column over all other rows in the table, or 1 if the tabular array is empty. Or, if the largest existing integer key 9223372036854775807 is in utilise and then an unused fundamental value is called at random. For example, suppose you lot have a tabular array like this:

CREATE Table t1(   a INTEGER PRIMARY Key,   b INTEGER );          

With this tabular array, the statement

INSERT INTO t1 VALUES(NULL,123);          

is logically equivalent to proverb:

INSERT INTO t1 VALUES((SELECT max(a) FROM t1)+1,123);          

At that place is a part named sqlite3_last_insert_rowid() which will render the integer fundamental for the most contempo insert operation.

Note that the integer key is 1 greater than the largest key that was in the table just prior to the insert. The new cardinal volition be unique over all keys currently in the tabular array, but it might overlap with keys that have been previously deleted from the tabular array. To create keys that are unique over the lifetime of the table, add the AUTOINCREMENT keyword to the INTEGER Master KEY announcement. Then the key chosen will exist 1 more than the largest key that has ever existed in that table. If the largest possible primal has previously existed in that tabular array, then the INSERT will neglect with an SQLITE_FULL error code.

(ii) What datatypes does SQLite support?

SQLite uses dynamic typing. Content tin can be stored as INTEGER, Existent, TEXT, BLOB, or every bit NULL.

(3) SQLite lets me insert a string into a database cavalcade of type integer!

This is a characteristic, non a bug. SQLite uses dynamic typing. It does non enforce data type constraints. Data of any type can (normally) be inserted into any column. Yous can put arbitrary length strings into integer columns, floating point numbers in boolean columns, or dates in graphic symbol columns. The datatype y'all assign to a cavalcade in the CREATE TABLE command does not restrict what data tin be put into that cavalcade. Every column is able to agree an capricious length cord. (There is one exception: Columns of type INTEGER Master Central may only concur a 64-bit signed integer. An fault will consequence if yous try to put annihilation other than an integer into an INTEGER PRIMARY Central column.)

Only SQLite does employ the alleged type of a cavalcade as a hint that you prefer values in that format. Then, for example, if a cavalcade is of type INTEGER and y'all try to insert a string into that cavalcade, SQLite will try to convert the string into an integer. If it can, information technology inserts the integer instead. If non, it inserts the cord. This feature is called type analogousness.

(4) Why doesn't SQLite allow me to use '0' and '0.0' as the primary fundamental on two different rows of the same table?

This trouble occurs when your chief fundamental is a numeric type. Modify the datatype of your primary primal to TEXT and information technology should piece of work.

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

(v) Can multiple applications or multiple instances of the same application access a single database file at the same time?

Multiple processes tin have the aforementioned database open up at the same fourth dimension. Multiple processes can be doing a SELECT at the same time. But only one procedure can be making changes to the database at any moment in time, however.

SQLite uses reader/author locks to control admission 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 machinery 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 piece of work under Fatty filesystems if you are non running the Share.exe daemon. People who take a lot of feel with Windows tell me that file locking of network files is very buggy and is not dependable. If what they say is truthful, sharing an SQLite database betwixt two or more than Windows machines might cause unexpected problems.

We are aware of no other embedded SQL database engine that supports as much concurrency as SQLite. SQLite allows multiple processes to have the database file open up at once, and for multiple processes to read the database at once. When any process wants to write, it must lock the unabridged database file for the elapsing of its update. But that unremarkably but takes a few milliseconds. Other processes only wait on the writer to finish then keep about their business. Other embedded SQL database engines typically only allow a single process to connect to the database at once.

All the same, 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 customer/server database because there is always a single well-controlled server procedure available to coordinate admission. If your awarding has a need for a lot of concurrency, then yous should consider using a client/server database. Merely feel suggests that nigh applications need much less concurrency than their designers imagine.

When SQLite tries to access a file that is locked by another process, the default beliefs is to render SQLITE_BUSY. You tin can accommodate this beliefs from C code using the sqlite3_busy_handler() or sqlite3_busy_timeout() API functions.

(6) Is SQLite threadsafe?

Threads are evil. Avoid them.

SQLite is threadsafe. Nosotros brand this concession since many users cull to ignore the communication given in the previous paragraph. But in lodge to exist thread-safe, SQLite must be compiled with the SQLITE_THREADSAFE preprocessor macro set to one. 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 confronting is compiled to be threadsafe you can call the sqlite3_threadsafe() interface to find out.

SQLite is threadsafe because information technology uses mutexes to serialize access to mutual information structures. However, the work of acquiring and releasing these mutexes will tiresome SQLite down slightly. Hence, if y'all do non demand SQLite to be threadsafe, you should disable the mutexes for maximum functioning. Run into the threading fashion documentation for boosted information.

Under Unix, y'all should not acquit an open SQLite database beyond a fork() system call into the kid process.

(7) How do I listing all tables/indices contained in an SQLite database

If y'all are running the sqlite3 command-line admission programme you can type ".tables" to get a list of all tables. Or you can type ".schema" to encounter the complete database schema including all tables and indices. Either of these commands can be followed by a Similar pattern that will restrict the tables that are displayed.

From inside a C/C++ program (or a script using Tcl/Ruby/Perl/Python bindings) yous can get access to table and index names by doing a SELECT on a special table named "SQLITE_SCHEMA". Every SQLite database has an SQLITE_SCHEMA table that defines the schema for the database. The SQLITE_SCHEMA table looks like this:

CREATE TABLE sqlite_schema (   type TEXT,   proper noun TEXT,   tbl_name TEXT,   rootpage INTEGER,   sql TEXT );          

For tables, the type field will e'er exist 'table' and the name field volition be the proper noun of the tabular array. So to get a list of all tables in the database, use the following SELECT command:

SELECT name FROM sqlite_schema WHERE blazon='table' Guild BY name;          

For indices, type is equal to 'index', name is the name of the index and tbl_name is the name of the table to which the index belongs. For both tables and indices, the sql field is the text of the original CREATE TABLE or CREATE Alphabetize statement that created the table or index. For automatically created indices (used to implement the Primary KEY or UNIQUE constraints) the sql field is NULL.

The SQLITE_SCHEMA tabular array cannot be modified using UPDATE, INSERT, or DELETE (except under extraordinary conditions). The SQLITE_SCHEMA tabular array is automatically updated by commands like CREATE Table, CREATE Index, DROP Table, and Drop INDEX.

Temporary tables exercise not appear in the SQLITE_SCHEMA table. Temporary tables and their indices and triggers occur in another special tabular array named SQLITE_TEMP_SCHEMA. SQLITE_TEMP_SCHEMA works just like SQLITE_SCHEMA except that it is only visible to the awarding that created the temporary tables. To get a list of all tables, both permanent and temporary, one can utilize a control like to the post-obit:

SELECT proper noun FROM     (SELECT * FROM sqlite_schema UNION ALL     SELECT * FROM sqlite_temp_schema) WHERE type='tabular array' ORDER BY name          

(8) Are there whatever known size limits to SQLite databases?

See limits.html for a full discussion of the limits of SQLite.

(9) What is the maximum size of a VARCHAR in SQLite?

SQLite does non enforce the length of a VARCHAR. Yous can declare a VARCHAR(ten) and SQLite will be happy to shop a 500-million character cord there. And it volition proceed all 500-million characters intact. Your content is never truncated. SQLite understands the column blazon of "VARCHAR(N)" to be the same as "TEXT", regardless of the value of North.

(x) Does SQLite back up a BLOB type?

SQLite allows you to store BLOB data in whatever column, even columns that are declared to concord some other type. BLOBs tin can even be used as Main KEYs.

(eleven) How do I add, delete or rename columns from an existing table in SQLite?

SQLite has limited Alter Table support that you tin use to add, rename or drop columns or to modify the proper name of a table as detailed at ALTER Table.

If y'all want to brand more complex changes in the structure or constraints of a tabular array or its columns, you will have to recreate it. You can salve existing data to a temporary table, drib the old tabular array, create the new tabular array, then copy the information back in from the temporary table. See Making Other Kinds Of Table Schema Changes for procedure.

(12) I deleted a lot of data merely the database file did not get any smaller. Is this a bug?

No. When y'all 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 information. The disk space is not lost. But neither is information technology returned to the operating system.

If you delete a lot of data and desire to shrink the database file, run the VACUUM command. VACUUM will reconstruct the database from scratch. This will leave the database with an empty gratuitous-list and a file that is minimal in size. Annotation, however, that the VACUUM can take some time to run and it tin use up to twice every bit much temporary deejay space equally the original file while it is running.

An alternative to using the VACUUM command is auto-vacuum manner, enabled using the auto_vacuum pragma.

(xiii) Can I use SQLite in my commercial product without paying royalties?

Yes. SQLite is in the public domain. No claim of ownership is made to any role of the code. You tin do anything you want with it.

(fourteen) How do I employ a string literal that contains an embedded single-quote (') character?

The SQL standard specifies that unmarried-quotes in strings are escaped past putting two single quotes in a row. SQL works like the Pascal programming linguistic communication in this regard. Case:

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

(xv) What is an SQLITE_SCHEMA error, and why am I getting one?

An SQLITE_SCHEMA error is returned when a prepared SQL argument is no longer valid and cannot exist executed. When this occurs, the argument 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. Yous volition never receive an SQLITE_SCHEMA mistake from sqlite3_exec(). Nor will you receive an error if you prepare statements using sqlite3_prepare_v2() instead of sqlite3_prepare().

The sqlite3_prepare_v2() interface creates a prepared argument that will automatically recompile itself if the schema changes. The easiest way to deal with SQLITE_SCHEMA errors is to ever use sqlite3_prepare_v2() instead of sqlite3_prepare().

(17) I get some compiler warnings when I compile SQLite. Isn't this a problem? Doesn't it indicate poor lawmaking quality?

Quality balls in SQLite is done using full-coverage testing, not by compiler warnings or other static lawmaking 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 of operations is devoted purely to testing. The SQLite exam suite runs tens of thousands of divide test cases and many of those test cases are parameterized and so that hundreds of millions of tests involving billions of SQL statements are run and evaluated for definiteness prior to every release. The developers use code coverage tools to verify that all paths through the code are tested. Whenever a bug is constitute in SQLite, new examination cases are written to exhibit the problems and then that the bug cannot recur undetected in the futurity.

During testing, the SQLite library is compiled with special instrumentation that allows the exam scripts to simulate a wide diversity of failures in social club to verify that SQLite recovers correctly. Memory resource allotment is carefully tracked and no memory leaks occur, fifty-fifty following retentivity allocation failures. A custom VFS layer is used to simulate operating organization crashes and power failures in guild to ensure that transactions are atomic beyond 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!)

Nosotros likewise run SQLite using Valgrind on Linux and verify that it detects no problems.

Some people say that nosotros should eliminate all warnings because beneficial warnings mask existent warnings that might ascend in time to come changes. This is true enough. But in respond, the developers observe that all warnings accept already been stock-still in the builds used for SQLite development (diverse versions of GCC, MSVC, and clang). Compiler warnings usually only arise from compilers or compile-fourth dimension options that the SQLite developers do not utilize themselves.

(xviii) Example-insensitive matching of Unicode characters does not work.

The default configuration of SQLite merely supports case-insensitive comparisons of ASCII characters. The reason for this is that doing full Unicode example-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 awarding that needs full Unicode instance back up probably already has the necessary tables and functions and so SQLite should not accept upward infinite to duplicate this power.

Instead of providing total 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 congenital-in similar(), 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-enlightened comparison routines already contained within their projection.

(19) INSERT is really tiresome - I can only do few dozen INSERTs per 2d

Actually, SQLite volition easily do 50,000 or more INSERT statements per second on an average desktop estimator. But information technology will merely do a few dozen transactions per 2nd. Transaction speed is limited by the rotational speed of your disk bulldoze. A transaction normally requires 2 complete rotations of the disk platter, which on a 7200RPM disk drive limits y'all to most sixty transactions per second.

Transaction speed is limited by disk bulldoze speed because (by default) SQLite actually waits until the data actually is safely stored on the disk surface earlier the transaction is complete. That manner, if y'all suddenly lose power or if your OS crashes, your information is still safe. For details, read most atomic commit in SQLite..

By default, each INSERT statement is its own transaction. But if you surround multiple INSERT statements with BEGIN...COMMIT and so 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 fourth dimension per insert statement is greatly reduced.

Another option is to run PRAGMA synchronous=OFF. This command will cause SQLite to non await on information to reach the disk surface, which volition make write operations appear to be much faster. Only if you lot lose power in the middle of a transaction, your database file might go corrupt.

(xx) I accidentally deleted some important information from my SQLite database. How can I recover it?

If you have a backup re-create of your database file, recover the information from your backup.

If you lot practise not take a backup, recovery is very difficult. You might be able to notice partial string data in a binary dump of the raw database file. Recovering numeric information might also be possible given special tools, though to our knowledge no such tools exist. SQLite is sometimes compiled with the SQLITE_SECURE_DELETE choice which overwrites all deleted content with zeros. If that is the case then recovery is clearly incommunicable. Recovery is also impossible if yous accept 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 withal exist in the database file, in areas marked for reuse. But, again, in that location exist no procedures or tools that we know of to assist yous recover that data.

(21) What is an SQLITE_CORRUPT error? What does it mean for the database to be "malformed"? Why am I getting this error?

An SQLITE_CORRUPT error is returned when SQLite detects an error in the structure, format, or other control elements of the database file.

SQLite does not decadent database files without external assist. If your application crashes in the middle of an update, your information is safe. The database is safe fifty-fifty if your Os crashes or takes a power loss. The crash-resistance of SQLite has been extensively studied and tested and is attested past years of real-world feel by billions of users.

That said, there are a number of things that external programs or bugs in your hardware or Os can exercise to corrupt a database file. See How To Corrupt An SQLite Database File for further information.

You tin can use PRAGMA integrity_check to do a thorough just time intensive test of the database integrity.

You can use PRAGMA quick_check to do a faster but less thorough test of the database integrity.

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, one time humpty-dumpty falls off the wall, it is mostly not possible to put him dorsum together once more.

(22) Does SQLite back up strange keys?

Every bit of version three.half-dozen.19 (2009-10-14), SQLite supports foreign key constraints. Simply enforcement of foreign primal constraints is turned off by default (for backwards compatibility). To enable foreign key constraint enforcement, run PRAGMA foreign_keys=ON or compile with -DSQLITE_DEFAULT_FOREIGN_KEYS=1.

(23) I get a compiler error if I apply the SQLITE_OMIT_... compile-time options when edifice SQLite.

The SQLITE_OMIT_... compile-fourth dimension options only work when building from canonical source files. They practice not work when y'all build from the SQLite affiliation or from the pre-processed source files.

It is possible to build a special amalgamation that volition work with a predetermined set of SQLITE_OMIT_... options. Instructions for doing so can be found with the SQLITE_OMIT_... documentation.

(24) My WHERE clause expression column1="column1" 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: column1='column1'

SQL uses double-quotes around identifiers (column or table names) that contains special characters or which are keywords. So double-quotes are a fashion of escaping identifier names. Hence, when y'all say column1="column1" that is equivalent to column1=column1 which is obviously always true.

(25) How are the syntax diagrams (a.one thousand.a. "railroad" diagrams) for SQLite generated?

Each diagram is hand-written using the Pikchr diagramming language. These hand-written specifications are converted into SVG and inserted inline in the HTML files as office of the documentation build process.

Many historical versions of the SQLite documentation used a different process for generating the syntax diagrams. The historical procedure was based on Tcl/Tk and is described at http://wiki.tcl-lang.org/21708. The newer Pikchr-based syntax diagrams first landed on trunk on 2020-09-26.

(26) The SQL standard requires that a UNIQUE constraint be enforced even if one or more than of the columns in the constraint are NULL, but SQLite does not exercise this. Isn't that a problems?

Perhaps you are referring to the following argument from SQL92:

A unique constraint is satisfied if and only if no ii rows in a tabular array have the same not-null values in the unique columns.

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

1. A unique constraint is satisfied if and only if no 2 rows in a tabular array have the aforementioned values and have not-nada values in the unique columns.
2. A unique constraint is satisfied if and only if no two rows in a table accept the same values in the subset of unique columns that are non zilch.

SQLite follows interpretation (i), as does PostgreSQL, MySQL, Oracle, and Firebird. It is true that Informix and Microsoft SQL Server use estimation (2), however nosotros the SQLite developers hold that estimation (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 besides get with (ane), then that is what SQLite does.

(27) What is the Consign Control Classification Number (ECCN) for SQLite?

Afterwards careful review of the Commerce Command List (CCL), we are convinced that the core public-domain SQLite source code is not described past whatsoever ECCN, hence the ECCN should be reported equally EAR99.

The in a higher place is true for the core public-domain SQLite. If you lot extend SQLite by adding new lawmaking, or if you lot statically link SQLite with your application, that might change the ECCN in your particular instance.

(28) My query does non return the column proper name that I look. Is this a bug?

If the columns of your result ready are named past AS clauses, then SQLite is guaranteed to utilise the identifier to the correct of the As keyword as the column proper name. If the result set up does not use an AS clause, and so SQLite is gratuitous to name the cavalcade anything information technology wants. See the sqlite3_column_name() documentation for further information.

Reading Column Data Instead of Rows Sqlite

Source: https://www.sqlite.org/faq.html

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