Text Table support for HSQLDB was originally developed by Bob Preston independently from the Project. Subsequently Bob joined the Project and incorporated this feature into version 1.7.0, with a number of enhancements, especially the use of conventional SQL commands for specifying the files used for Text Tables.
In a nutshell, Text Tables are CSV or other delimited files treated as SQL tables. Any ordinary CSV or other delimited file can be used. The full range of SQL queries can be performed on these files, including SELECT, INSERT, UPDATE and DELETE. Indexes and unique constraints can be set up, and foreign key constraints can be used to enforce referential integrity between Text Tables themselves or with conventional tables.
HSQLDB with Text Table support is the only comprehensive solution that employs the power of SQL and the universal reach of JDBC to handle data stored in text files and will have wide ranging use way beyond the currently established Java realm of HSQLDB.
Text Tables are defined similarly to conventional tables with the added TEXT keyword:
CREATE [TEMP] TEXT TABLE <tablename> (<column definition>… [<constraint definition>…])
In addition, a SET command specifies the file and the separator character that the Text table uses:
SET TABLE <tablename> SOURCE <quoted_filename_and_options> [DESC]
Text Tables cannot be created in memory-only databases (databases that have no script file).
The default field separator is a comma (,). A different field separator can be specified in either the <databasename>.properties file or within the SET TABLE SOURCE statement. For example, to change the field separator for the table mytable to a vertical bar, either place the following in the <databasename>.properties file:
textdb.mytable.fs=|
Or, within the SET TABLE SOURCE statement, for example:
SET TABLE mytable SOURCE "myfile;fs=|"
Since HSQLDB treats CHAR’s, VARCHAR’s, and LONGVARCHAR’s the same, the ability to assign different separators to the latter two is provided. When a different separator is assigned to a VARCHAR or LONGVARCHAR field, it will terminate any CSV field of that type. For example, if the first field is CHAR, and the second field LONGVARCHAR, and the separator fs has been defined as the pipe (|) and vs as the period (.) then the data in the CSV file for a row will look like:
First field data|Second field data.Third field data
The following example shows how to change the default separator to the pipe (|), VARCHAR separator to the period (.) and the LONGVARCHAR separator to the tilde (~). Either place the following in the .properties file:
textdb.mytable.fs=| textdb.mytable.vs=. textdb.mytable.lvs=~
Or, within the SET TABLE SOURCE statement, for example:
SET TABLE mytable SOURCE "myfile;fs=|;vs=.;lvs=~"
HSQLDB also recognises the following special indicators for separators:
\semi - semicolon
\quote - quote
\space - space character
\apos - apostrophe
\n - newline - Used as an end anchor (like $ in regular expressions)
\r - carriage return
\t - tab
\\ - backslash
\u#### - a Unicode character specified in hexadecimal
Furthermore, HSQLDB provides csv file support with two additional boolean options: ignore_first and quoted. The ignore_first option (default false) tells HSQLDB to ignore the first line in a file. This option is used when the first line of the file contains column headings. The quoted option (default true) tells the program that some fields may be quoted. These options may also be specified in the .properties file:
textdb.mytable.ignore_first=true textdb.mytable.quoted=false
Or, within the SET TABLE SOURCE statement:
SET TABLE mytable SOURCE "myfile;ignore_first=true;quoted=false"
When the default quoted option is in force, fields that are written to a line of the csv file will be quoted only if they contain the separator character.
Finally, HSQLDB provides the ability to read a text file from the bottom up, by placing the keyword "DESC" at the end of the SET TABLE SOURCE statement:
SET TABLE mytable SOURCE "myfile" DESC
This feature provides functionality similar to the Unix tail command, by re-reading the file each time a select is executed. Using this feature sets the table to read-only mode. Afterwards, it will no longer be possible to change the read-only status with SET TABLE <tablename> READONLY TRUE. [Does this feature allow a different program to modify the csv file from time to time while it is being used by HSQLDB?]
textdb.<tablename>.allow_full_path=true.
SELECT <select list> INTO TEXT <tablename> FROM …
is the directory that contains the database and the file name is based on the table name. The table name is converted into the file name by replacing all the non-alphanumeric characters with the underscore character, conversion into lowercase, and adding the ".csv" suffix.
textdb.mytable.ignore_first=true
option, the first, ignored line is deleted after updating the table and replaced with a blank line.Copyright 2002 Bob Preston and Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQLDB Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.