ที่มา : http://www.sqlite.org/sqlite.html
The SQLite library includes a simple command-line utility named sqlite3 (or sqlite3.exe on windows) that allows the user to manually enter and execute SQL commands against an SQLite database. This document provides a brief introduction on how to use the sqlite3 program.
Getting Started
To start the sqlite3 program, just type “sqlite3” followed by the name the file that holds the SQLite database. If the file does not exist, a new one is created automatically. The sqlite3program will then prompt you to enter SQL. Type in SQL statements (terminated by a semicolon), press “Enter” and the SQL will be executed.
For example, to create a new SQLite database named “ex1” with a single table named “tbl1”, you might do this:
You can terminate the sqlite3 program by typing your systems End-Of-File character (usually a Control-D). Use the interrupt character (usually a Control-C) to stop a long-running SQL statement.
Make sure you type a semicolon at the end of each SQL command! The sqlite3 program looks for a semicolon to know when your SQL command is complete. If you omit the semicolon, sqlite3 will give you a continuation prompt and wait for you to enter more text to be added to the current SQL command. This feature allows you to enter SQL commands that span multiple lines. For example:
Aside: Querying the SQLITE_MASTER table
The database schema in an SQLite database is stored in a special table named “sqlite_master”. You can execute “SELECT” statements against the special sqlite_master table just like any other table in an SQLite database. For example:
But you cannot execute DROP TABLE, UPDATE, INSERT or DELETE against the sqlite_master table. The sqlite_master table is updated automatically as you create or drop tables and indices from the database. You can not make manual changes to the sqlite_master table.
The schema for TEMPORARY tables is not stored in the “sqlite_master” table since TEMPORARY tables are not visible to applications other than the application that created the table. The schema for TEMPORARY tables is stored in another special table named “sqlite_temp_master”. The “sqlite_temp_master” table is temporary itself.
Special commands to sqlite3
Most of the time, sqlite3 just reads lines of input and passes them on to the SQLite library for execution. But if an input line begins with a dot (“.”), then that line is intercepted and interpreted by the sqlite3 program itself. These “dot commands” are typically used to change the output format of queries, or to execute certain prepackaged query statements.
For a listing of the available dot commands, you can enter “.help” at any time. For example:
Changing Output Formats
The sqlite3 program is able to show the results of a query in eight different formats: “csv”, “column”, “html”, “insert”, “line”, “list”, “tabs”, and “tcl”. You can use the “.mode” dot command to switch between these output formats.
The default output mode is “list”. In list mode, each record of a query result is written on one line of output and each column within that record is separated by a specific separator string. The default separator is a pipe symbol (“|”). List mode is especially useful when you are going to send the output of a query to another program (such as AWK) for additional processing.
You can use the “.separator” dot command to change the separator for list mode. For example, to change the separator to a comma and a space, you could do this:
In “line” mode, each column in a row of the database is shown on a line by itself. Each line consists of the column name, an equal sign and the column data. Successive records are separated by a blank line. Here is an example of line mode output:
In column mode, each record is shown on a separate line with the data aligned in columns. For example:
By default, each column is at least 10 characters wide. Data that is too wide to fit in a column is truncated. You can adjust the column widths using the “.width” command. Like this:
The “.width” command in the example above sets the width of the first column to 12 and the width of the second column to 6. All other column widths were unaltered. You can gives as many arguments to “.width” as necessary to specify the widths of as many columns as are in your query results.
If you specify a column a width of 0, then the column width is automatically adjusted to be the maximum of three numbers: 10, the width of the header, and the width of the first row of data. This makes the column width self-adjusting. The default width setting for every column is this auto-adjusting 0 value.
The column labels that appear on the first two lines of output can be turned on and off using the “.header” dot command. In the examples above, the column labels are on. To turn them off you could do this:
Another useful output mode is “insert”. In insert mode, the output is formatted to look like SQL INSERT statements. You can use insert mode to generate text that can later be used to input data into a different database.
When specifying insert mode, you have to give an extra argument which is the name of the table to be inserted into. For example:
The last output mode is “html”. In this mode, sqlite3 writes the results of the query as an XHTML table. The beginning <TABLE> and the ending </TABLE> are not written, but all of the intervening <TR>s, <TH>s, and <TD>s are. The html output mode is envisioned as being useful for CGI.
Writing results to a file
By default, sqlite3 sends query results to standard output. You can change this using the “.output” command. Just put the name of an output file as an argument to the .output command and all subsequent query results will be written to that file. Use “.output stdout” to begin writing to standard output again. For example:
Querying the database schema
The sqlite3 program provides several convenience commands that are useful for looking at the schema of the database. There is nothing that these commands do that cannot be done by some other means. These commands are provided purely as a shortcut.
For example, to see a list of the tables in the database, you can enter “.tables”.
The “.tables” command is similar to setting list mode then executing the following query:
In fact, if you look at the source code to the sqlite3 program (found in the source tree in the file src/shell.c) you’ll find exactly the above query.
The “.indices” command works in a similar way to list all of the indices for a particular table. The “.indices” command takes a single argument which is the name of the table for which the indices are desired. Last, but not least, is the “.schema” command. With no arguments, the “.schema” command shows the original CREATE TABLE and CREATE INDEX statements that were used to build the current database. If you give the name of a table to “.schema”, it shows the original CREATE statement used to make that table and all if its indices. We have:
The “.schema” command accomplishes the same thing as setting list mode, then entering the following query:
Or, if you give an argument to “.schema” because you only want the schema for a single table, the query looks like this:
You can supply an argument to the .schema command. If you do, the query looks like this:
The “%s” in the query is replace by your argument. This allows you to view the schema for some subset of the database.
Along these same lines, the “.table” command also accepts a pattern as its first argument. If you give an argument to the .table command, a “%” is both appended and prepended and a LIKE clause is added to the query. This allows you to list only those tables that match a particular pattern.
The “.databases” command shows a list of all databases open in the current connection. There will always be at least 2. The first one is “main”, the original database opened. The second is “temp”, the database used for temporary tables. There may be additional databases listed for databases attached using the ATTACH statement. The first output column is the name the database is attached with, and the second column is the filename of the external file.
Converting An Entire Database To An ASCII Text File
Use the “.dump” command to convert the entire contents of a database into a single ASCII text file. This file can be converted back into a database by piping it back into sqlite3.
A good way to make an archival copy of a database is this:
This generates a file named ex1.dump.gz that contains everything you need to reconstruct the database at a later time, or on another machine. To reconstruct the database, just type:
The text format is pure SQL so you can also use the .dump command to export an SQLite database into other popular SQL database engines. Like this:
Other Dot Commands
The “.explain” dot command can be used to set the output mode to “column” and to set the column widths to values that are reasonable for looking at the output of an EXPLAIN command. The EXPLAIN command is an SQLite-specific SQL extension that is useful for debugging. If any regular SQL is prefaced by EXPLAIN, then the SQL command is parsed and analyzed but is not executed. Instead, the sequence of virtual machine instructions that would have been used to execute the SQL command are returned like a query result. For example:
The “.timeout” command sets the amount of time that the sqlite3 program will wait for locks to clear on files it is trying to access before returning an error. The default value of the timeout is zero so that an error is returned immediately if any needed database table or index is locked.
And finally, we mention the “.exit” command which causes the sqlite3 program to exit.
Using sqlite3 in a shell script
One way to use sqlite3 in a shell script is to use “echo” or “cat” to generate a sequence of commands in a file, then invoke sqlite3 while redirecting input from the generated command file. This works fine and is appropriate in many circumstances. But as an added convenience, sqlite3 allows a single SQL command to be entered on the command line as a second argument after the database name. When the sqlite3 program is launched with two arguments, the second argument is passed to the SQLite library for processing, the query results are printed on standard output in list mode, and the program exits. This mechanism is designed to make sqlite3 easy to use in conjunction with programs like “awk”. For example:
Ending shell commands
SQLite commands are normally terminated by a semicolon. In the shell you can also use the word “GO” (case-insensitive) or a slash character “/” on a line by itself to end a command. These are used by SQL Server and Oracle, respectively. These won’t work in sqlite3_exec(), because the shell translates these into a semicolon before passing them to that function.
Compiling the sqlite3 program from sources
The source code to the sqlite3 command line interface is in a single file named “shell.c” which you can download from the SQLite website. Compile this file (together with the sqlite3 library source code to generate the executable. For example: