tdbc::connection(n) Tcl Database Connectivity tdbc::connection(n)______________________________________________________________________________NAME
tdbc::connection - TDBC connection object
SYNOPSIS
package require tdbc 1.0
package require tdbc::driver version
tdbc::driver::connection create db ?-option value...?
db configure ?-option value...?
db close
db prepare sql-code
db preparecall call
db statements
db resultsets
db tables ?pattern?
db columns table ?pattern?
db begintransaction
db commit
db rollback
db transaction script
db allrows ?-as lists|dicts? ?-columnsvariable name? ?--? sql-code
?dictionary?
db foreach ?-as lists|dicts? ?-columnsvariable name? ?--? sql-code
?dictionary? varName script
_DESCRIPTION
Every database driver for TDBC (Tcl DataBase Connectivity) implements a
connection object that represents a connection to a database. By con‐
vention, this object is created by the command, tdbc::driver::connec‐
tion create. This command accepts the name of a Tcl command that will
represent the connection and a possible set of options (see CONFIGURA‐
TION OPTIONS). It establishes a connection to the database and returns
the name of the newly-created Tcl command.
The configure object command on a database connection, if presented
with no arguments, returns a list of alternating keywords and values
representing the connection's current configuration. If presented with
a single argument -option, it returns the configured value of the given
option. Otherwise, it must be given an even number of arguments which
are alternating options and values. The specified options receive the
specified values, and nothing is returned.
The close object command on a database connection closes the connec‐
tion. All active statements and result sets on the connection are
closed. Any uncommitted transaction is rolled back. The object command
is deleted.
The prepare object command on a database connection prepares a SQL
statement for execution. The sql-code argument must contain a single
SQL statement to be executed. Bound variables may be included. See SQL
STATEMENTS for a further description of the SQL that is accepted by
this command. The return value is a newly-created Tcl command that rep‐
resents the statement. See tdbc_statement(n) for the interface accepted
by a statement.
On a database connection where the underlying database and driver sup‐
port stored procedures, the preparecall object command prepares a call
to a stored procedure for execution. The syntax of the stored proce‐
dure call is:
?resultvar =? procname(?arg ?, arg...?)
The return value is a newly-created Tcl command that represents the
statement. See tdbc_statement(n) for the interface accepted by a state‐
ment.
The statements object command returns a list of statements that have
been created by prepare and preparecall statements against the given
connection and have not yet been closed.
The resultsets object command returns a list of result sets that have
been obtained by executing statements prepared using the given connec‐
tion and not yet closed.
The tables object command allows the program to query the connection
for the names of tables that exist in the database. The optional pat‐
tern parameter is a pattern to match the name of a table. It may con‐
tain the SQL wild-card characters '%' and and whose values are subdic‐
tionaries. See the documentation for the individual database driver for
the interpretation of the values.
The columns object command allows the program to query the connection
for the names of columns that exist in a given table. The optional
pattern parameter is a pattern to match the name of a column. It may
contain the SQL wild-card characters '%' and and whose values are dic‐
tionaries. Each of the subdictionaries will contain at least the fol‐
lowing keys and values (and may contain others whose usage is deter‐
mined by a specific database driver).
type Contains the data type of the column, and will generally be cho‐
sen from the set, bigint, binary, bit, char, date, decimal, dou‐
ble, float, integer, longvarbinary, longvarchar, numeric, real,
time, timestamp, smallint, tinyint, varbinary, and varchar. (If
the column has a type that cannot be represented as one of the
above, type will contain a driver-dependent description of the
type.)
precision
Contains the precision of the column in bits, decimal digits, or
the width in characters, according to the type.
scale Contains the scale of the column (the number of digits after the
radix point), for types that support the concept.
nullable
Contains 1 if the column can contain NULL values, and 0 other‐
wise.
The starttransaction object command on a database connection begins a
transaction on the database. If the underlying database does not sup‐
port atomic, consistent, isolated, durable transactions, the start‐
transaction object command returns an error reporting the fact. Simi‐
larly, if multiple starttransaction commands are executed withough an
intervening commit or rollback command, an error is returned unless the
underlying database supports nested transactions.
The commit object command on a database connection ends the most recent
transaction started by starttransaction and commits changes to the
database.
The rollback object command on a database connection rolls back the
most recent transaction started by starttransaction. The state of the
database is as if nothing happened during the transaction.
The transaction object command on a database connection presents a sim‐
ple way of bundling a database transaction. It begins a transaction,
and evaluates the supplied script argument as a Tcl script in the call‐
er's scope. If script terminates normally, or by break, continue, or
return, the transaction is committed (and any action requested by
break, continue, or return takes place). If the evaluation of script
ends with an error, the transaction is rolled back and the error is
rethrown. Any other return code is treated similarly to break, con‐
tinue, or return.
The allrows object command prepares a SQL statement (given by the sql-
code parameter) to execute against the database. It then executes it
(see tdbc_statement(n) for details) with the optional dictionary param‐
eter giving bind variables. Finally, it uses the allrows object command
on the result set (see tdbc_resultset(n)) to construct a list of the
results. Finally, both result set and statement are closed. The return
value is the list of results.
The foreach object command prepares a SQL statement (given by the sql-
code parameter) to execute against the database. It then executes it
(see tdbc_statement(n) for details) with the optional dictionary param‐
eter giving bind variables. Finally, it uses the foreach object command
on the result set (see tdbc_resultset(n)) to evaluate the given script
for each row of the results. FinRlresultshineaureturn,aandserror,noraan
closed, even if the given script
unusual return code.
CONFIGURATION OPTIONS
The configuration options accepted when the connection is created and
on the connection's configure object command include the following, and
may include others specific to a database driver.
-encoding name
Specifies the encoding to be used in connecting to the database.
The name should be one of the names accepted by the encoding(n)
command. This option is usually unnecessary; most database driv‐
ers can figure out the encoding in use by themselves.
-isolation level
Specifies the transaction isolation level needed for transac‐
tions on the database. The acceptable values for level are shown
under TRANSACTION ISOLATION LEVELS.
-timeout ms
Specifies the maximum time to wait for a an operation database
engine before reporting an error to the caller. The ms argument
gives the maximum time in milliseconds. A value of zero (the
default) specifies that the calling process is to wait indefi‐
nitely for database operations.
-readonly flag
Specifies that the connection will not modify the database (if
the Boolean parameter flag is true), or that it may modify the
database (if flag is false). If flag is true, this option may
have the effect of raising the transaction isolation level to
readonly.
TRANSACTION ISOLATION LEVELS
The acceptable values for the -isolation configuration option are as
follows:
readuncommitted
Allows the transaction to read "dirty", that is, uncommitted
data. This isolation level may compromise data integrity, does
not guarantee that foreign keys or uniqueness constraints are
satisfied, and in general does not guarantee data consistency.
readcommitted
Forbids the transaction from reading "dirty" data, but does not
guarantee repeatable reads; if a transaction reads a row of a
database at a given time, there is no guarantee that the same
row will be available at a later time in the same transaction.
repeatableread
Guarantees that any row of the database, once read, will have
the same values for the life of a transaction. Still permits
"phantom reads" (that is, newly-added rows appearing if a table
is queried a second time).
serializable
The most restrictive (and most expensive) level of transaction
isolation. Any query to the database, if repeated, will return
precisely the same results for the life of the transaction,
exactly as if the transaction is the only user of the database.
readonly
Behaves like serializable in that the only results visible to
the transaction are those that were committed prior to the start
of the transaction, but forbids the transaction from modifying
the database.
A database that does not implement one of these isolation levels will
instead use the next more restrictive isolation level. If the given
level of isolation cannot be obtained, the database interface throws an
error reporting the fact. The default isolation level is readcommitted.
A script should not the isolation level when a transaction is in
progress.
SEE ALSOencoding(n), tdbc(n), tdbc_resultSet(n), tdbc_statement(n), tdbc_tok‐
enize(n)KEYWORDS
TDBC, SQL, database, connectivity, connection, resultset, statement
COPYRIGHT
Copyright (c) 2008 by Kevin B. Kenny.
Tcl 8.6 tdbc::connection(n)
