Returns an array containing all the SQLClient instances.

Return an existing SQLClient instance (using +existingClient:) if possible, or creates one, initialises it using -initWithConfiguration:name: , and returns the new instance (autoreleased).
Returns nil on failure.

Return an existing SQLClient instance for the specified name if one exists, otherwise returns nil.

Return the maximum number of simultaneous database connections permitted (set by +setMaxConnections: and defaults to 8)

Use this method to reduce the number of database connections currently active so that it is less than the limit set by the +setMaxConnections: method. This mechanism is used internally by the class to ensure that, when it is about to open a new connection, the limit is not exceeded.

If since is not nil, then any connection which has not been used more recently than that date is disconnected anyway.
You can (and probably should) use this periodically to purge idle connections, but you can also pass a date in the future to close all connections.

Set the maximum number of simultaneous database connections permitted (defaults to 8 and may not be set less than 1).

This value is used by the +purgeConnections: method to determine how many connections should be disconnected when it is called.

Start a transaction for this database client.
You must match this with either a -commit or a -rollback .

Normally, if you execute an SQL statement without using this method first, the autocommit feature is employed, and the statement takes effect immediately. Use of this method permits you to execute several statements in sequence, and only have them take effect (as a single operation) when you call the -commit method.

NB. You must not execute an SQL statement which would start a transaction directly... use only this method.

Where possible, consider using the SQLTransaction class rather than calling -begin -commit or -rollback yourself.

Return the client name for this instance.
Normally this is useful only for debugging/reporting purposes, but if you are using multiple instances of this class in your application, and you are using embedded SQL, you will need to use this method to fetch the client/connection name and store its C-string representation in a variable 'connectionName' declared to the sql preprocessor, so you can then have statements of the form - 'exec sql at :connectionName...'.

Complete a transaction for this database client.
This must match an earlier -begin .

NB. You must not execute an SQL statement which would commit or rollback a transaction directly... use only this method or the -rollback method.

Where possible, consider using the SQLTransaction class rather than calling -begin -commit or -rollback yourself.

If the connected instance variable is NO, this method calls -backendConnect to ensure that there is a connection to the database server established. Returns the result.
Performs any necessary locking for thread safety.

Return a flag to say whether a connection to the database server is currently live. This is mostly useful for debug/reporting, but is used internally to keep track of active connections.

Return the database name for this instance (or nil).

If the connected instance variable is YES, this method calls -backendDisconnect to ensure that the connection to the database server is dropped.
Performs any necessary locking for thread safety.

Perform arbitrary operation which does not return any value.
This arguments to this method are a nil terminated list which are concatenated in the manner of the -query:,... method.
Any string arguments are assumed to have been quoted appropriately already, but non-string arguments are automatically quoted using the -quote: method.

   [db execute: @"UPDATE ", table, @" SET Name = ",
     myName, " WHERE ID = ", myId, nil];
 

Takes the statement and substitutes in values from the dictionary where markup of the format {key} is found.
Passes the result to the -execute:,... method.

   [db execute: @"UPDATE {Table} SET Name = {Name} WHERE ID = {ID}"
          with: values];
 

Any non-string values in the dictionary will be replaced by the results of the -quote: method.
The markup format may also be {key?default} where default is a string to be used if there is no value for the key in the dictionary.

Calls -initWithConfiguration:name: passing a nil reference name.

Initialise using the supplied configuration, or if that is nil, try to use values from NSUserDefaults (and automatically update when the defaults change).
Uses the reference name to determine configuration information... and if a nil name is supplied, defaults to the value of SQLClientName in the configuration dictionary (or in the standard user defaults). If there is no value for SQLClientName, uses the string 'Database'.
If a SQLClient instance already exists with the name used for this instance, the receiver is deallocated and the existing instance is retained and returned... there may only ever be one instance for a particular reference name.

The config argument (or the SQLClientReferences user default) is a dictionary with names as keys and dictionaries as its values. Configuration entries from the dictionary corresponding to the database client are used if possible, general entries are used otherwise.
Database... is the name of the database to use, if it is missing then 'Database' may be used instead.
User... is the name of the database user to use, if it is missing then 'User' may be used instead.
Password... is the name of the database user password, if it is missing then 'Password' may be used instead.
missing then 'Password' may be used instead.
ServerType... is the name of the backend server to be used... by convention the name of a bundle containing the interface to that backend. If this is missing then 'Postgres' is used.

Return the state of the flag indicating whether the library thinks a transaction is in progress. This flag is normally maintained by -begin , -commit , and -rollback .

Returns the date/time stamp of the last database operation performed by the receiver, or nil if no operation has ever been done by it.
Simply connecting to or disconnecting from the databsse does not count as an operation.

Return the database reference name for this instance (or nil).

Return the database password for this instance (or nil).

Perform arbitrary query which returns values.

This method has at least one argument, the string starting the statement to be executed (which must have the prefix 'select ').

Additional arguments are a nil terminated list which also be strings, and these are appended to the statement.
Any string arguments are assumed to have been quoted appropriately already, but non-string arguments are automatically quoted using the -quote: method.

   result = [db query: @"SELECT Name FROM ", table, nil];
 

Upon error, an exception is raised.

The query returns an array of records (each of which is represented by an SQLRecord object).

Each SQLRecord object contains one or more fields, in the order in which they occurred in the query. Fields may also be retrieved by name.

NULL field items are returned as NSNull objects.

Most other field items are returned as NSString objects.

Date and timestamp field items are returned as NSDate objects.

Takes the query statement and substitutes in values from the dictionary where markup of the format {key} is found.
Passes the result to the -query:,... method to execute.

   result = [db query: @"SELECT Name FROM {Table} WHERE ID = {ID}"
                 with: values];
 

Any non-string values in the dictionary will be replaced by the results of the -quote: method.
The markup format may also be {key?default} where default is a string to be used if there is no value for the key in the dictionary.

Convert an object to a string suitable for use in an SQL query.
Normally the -execute:,... , and -query:,... methods will call this method automatically for everything apart from string objects.
Strings have to be handled specially, because they are used both for parts of the SQL command, and as values (where they need to be quoted). So where you need to pass a string value which needs quoting, you must call this method explicitly.
Subclasses may override this method to provide appropriate quoting for types of object which need database backend specific quoting conventions. However, the defalt implementation should be OK for most cases.
This method makes use of -quoteString: to quote literal strings.
The base class implementation formats NSDate objects as
YYYY-MM-DD hh:mm:ss.mmm ?ZZZZ
NSData objects are not quoted... they must not appear in queries, and where used for insert/update operations, they need to be passed to the -backendExecute: method unchanged.
For a nil object, we return NULL.
For a number, we simply convert directly to a string.
For a date, we convert to the text format used by the database, and add leading and trailing quotes.
For a data object, we don't quote... the other parts of the code need to know they have an NSData object and pass it on unchanged to the -backendExecute: method.
Just in case an NSNull subclass has been created by someone. The normal NSNull instance should have been handled earlier.
For an NSArray or NSSet, we produce a bracketed list of the (quoted) objects in the array.
For any other type of data, we just produce a quoted string representation of the objects description.

Convert a 'C' string to a string suitable for use in an SQL query by using -quoteString: to convert it to a literal string format.
NB. a null pointer is treated as an empty string.

Convert a single character to a string suitable for use in an SQL query by using -quoteString: to convert it to a literal string format.
NB. a nul character is not allowed and will cause an exception.

Convert a float to a string suitable for use in an SQL query.

Convert an integer to a string suitable for use in an SQL query.

Convert a string to a form suitable for use as a string literal in an SQL query.
Subclasses may override this for non-standard literal string quoting conventions.

Produce a quoted string from the supplied arguments (printf style).

Revert a transaction for this database client.
If there is no transaction in progress, this method does nothing.

NB. You must not execute an SQL statement which would commit or rollback a transaction directly... use only this method or the -rollback method.

Where possible, consider using the SQLTransaction class rather than calling -begin -commit or -rollback yourself.

Set the database host/name for this object.
This is called automatically to configure the connection... you normally shouldn't need to call it yourself.

Set the database reference name for this object. This is used to differentiate between multiple connections to the database.
This is called automatically to configure the connection... you normally shouldn't need to call it yourself.
NB. attempts to change the name of an instance to that of an existing instance are ignored.

Set the database password for this object.
This is called automatically to configure the connection... you normally shouldn't need to call it yourself.

Set the database user for this object.
This is called automatically to configure the connection... you normally shouldn't need to call it yourself.

Calls -backendExecute: in a safe manner.
Handles locking.
Maintains -lastOperation date.

Calls -backendQuery: in a safe manner.
Handles locking.
Maintains -lastOperation date.

Return the database user for this instance (or nil).

Create a new SQLRecord containing the specified fields.
NB. The values and keys are retained by the record rather than being copied.
A nil value is represented by [NSNull null].
Keys must be unique string values (case insensitive comparison).

Returns an array containing the names of all the fields in the record.

Return the record as a mutable dictionary with the keys as the record field names standardised to be lowercase strings.

Optimised mechanism for retrieving all objects.

Returns the value of the named field.
The field name is case insensitive.

Replaces the value of the named field.
The field name is case insensitive.
NB. You must be careful not to change the contents of a record which has been cached (unless you are sure you really want to), as you will be changing the contents of the cache, not just a private copy.

Return approximate size of this record in bytes.
The exclude set is used to specify objects to exclude from the calculation (to prevent recursion etc).

Adds an SQL statement to the transaction. This is similar to [SQLClient -execute:,...] but does not cause any database operation until -execute is called, so it will not raise a database exception.

Adds an SQL statement to the transaction. This is similar to [SQLClient -execute:with:] but does not cause any database operation until -execute is called, so it will not raise a database exception.

Appends all the statements from the other transaction to the receiver.
This provides a convenient way of merging transactions which have been built by different code modules, in order to have them all executed together in a single operation (for efficiency etc).
This does not alter the other transaction, so if the execution of a group of merged transactions fails, it is then possible to attempt to commit the individual transactions separately.
NB. All transactions appended must be using the same database connection (SQLClient instance).

Returns the number of statements in this transaction.

Returns the database client with which this instance operates.
This client is retained by the transaction.

Performs any statements added to the transaction as a single operation. If any problem occurs, an NSException is raised, but the database connection is left in a consistent state and a partially completed operation is rolled back.

NB. If the database is not already in a transaction, this implicitly calls the -begin method to start the transaction before executing the statements.
The method always commits the transaction, even if the transaction was begun earlier rather than in -execute .
This behavior allows you to call [SQLClient -begin] , then run one or more queries, build up a transaction based upon the query results, and then -execute that transaction, causing the entire process to be commited as a single transaction.

Resets the transaction, removing all previously added statements. This allows the transaction object to be re-used for multiple transactions.

Returns the cache used by the receiver for storing the results of requests made through it.

If the result of the query is already cached and is still valid, return it. Otherwise, perform the query and cache the result giving it the specified lifetime in seconds.
If seconds is negative, the query is performed irrespective of whether it is already cached, and its absolute value is used to set the lifetime of the results.
If seconds is zero, the cache for this query is emptied.

If the result of the query is already cached and is still valid, return it. Otherwise, perform the query and cache the result giving it the specified lifetime in seconds.
If seconds is negative, the query is performed irrespective of whether it is already cached, and its absolute value is used to set the lifetime of the results.
If seconds is zero, the cache for this query is emptied.

If the result of the query is already cached and is still valid, return it. Otherwise, perform the query and cache the result giving it the specified lifetime in seconds.
If seconds is negative, the query is performed irrespective of whether it is already cached, and its absolute value is used to set the lifetime of the results.
If seconds is zero, the cache for this query is emptied.
Handles locking.
Maintains -lastOperation date.

Sets the cache to be used by the receiver for storing the results of requests made through it.
If aCache is nil, the current cache is released, and a new cache will be automatically created as soon as there is a need to cache anything.

Executes a query (like the -query:,... method) and checks the result (raising an exception if the query did not contain a single record) and returns the resulting record.

Executes a query (like the -query:,... method) and checks the result.
Raises an exception if the query did not contain a single record, or if the record did not contain a single field.
Returns the resulting field as a string.

Convenience method to deal with the results of a query where each record contains a single field... it converts the array of records returned by the query to an array containing the fields.

Creates and returns an autoreleased SQLTransaction instance which will use the receiver as the database connection to perform transactions.

Return the class-wide debugging level, which is inherited by all newly created instances.

Return the class-wide duration logging threshold, which is inherited by all newly created instances.

Set the debugging level to be inherited by all new instances.
See [SQLClient(Logging) -setDebugging:] for controlling an individual instance of the class.

Set the duration logging threshold to be inherited by new instances.
See [SQLClient(Logging) -setDurationLogging:] for controlling an individual instance of the class.

The default implementation calls NSLogv to log a debug message.
Override this in a category to provide more sophisticated logging.

Return the current debugging level.
A level of zero (default) means that no debug output is produced, except for that concerned with logging the database transactions taking over a certain amount of time (see the -setDurationLogging: method).

Returns the threshold above which queries and statements taking a long time to execute are logged. A negative value (default) indicates that this logging is disabled. A value of zero means that all statements are logged.

Set the debugging level of this instance... overrides the default level inherited from the class.

Set a threshold above which queries and statements taking a long time to execute are logged. A negative value (default) disables this logging. A value of zero logs all statements.

Attempts to establish a connection to the database server.
Returns a flag to indicate whether the connection has been established.
If a connection was already established, returns YES and does nothing.
You should not need to use this method normally, as it is called for you automatically when necessary.

Subclasses must implement this method to establish a connection to the database server process (and initialise the extra instance variable if necessary), setting the connected instance variable to indicate the state of the object.

This method must call +purgeConnections: to ensure that there is a free slot for the new connection.

Application code must not call this method directly, it is for internal use only. The -connect method calls this method if the connected instance variable is NO.

Disconnect from the database unless already disconnected.

This method is called automatically when the receiver is deallocated or reconfigured, and may also be called automatically when there are too many database connections active.

If the receiver is an instance of a subclass which uses the extra instance variable, it must clear that variable in the -backendDisconnect method, because a reconfiguration may cause the class of the receiver to change.

This method must set the connected instance variable to NO.

Application code must not call this method directly, it is for internal use only. The -disconnect method calls this method if the connected instance variable is YES.

Perform arbitrary operation which does not return any value.
This method has a single argument, an array containing the string representing the statement to be executed as its first object, and an optional sequence of data objects following it.

   [db backendExecute: [NSArray arrayWithObject:
     @"UPDATE MyTable SET Name = 'The name' WHERE ID = 123"]];
 

The backend implementation is required to perform the SQL statement using the supplied NSData objects at the points in the statement marked by the '?'''?' sequence. The marker saequences are inserted into the statement at an earlier stage by the -execute:,... and -execute:with: methods.

This method should lock the instance using the lock instance variable for the duration of the operation, and unlock it afterwards.

NB. callers (other than the -begin , -commit , and -rollback methods) should not pass any statement to this method which would cause a transaction to begin or end.

Application code must not call this method directly, it is for internal use only.

Perform arbitrary query which returns values.

   result = [db backendQuery: @"SELECT Name FROM Table"];
 

Upon error, an exception is raised.

The query returns an array of records (each of which is represented by an SQLRecord object).

Each SQLRecord object contains one or more fields, in the order in which they occurred in the query. Fields may also be retrieved by name.

NULL field items are returned as NSNull objects.

This method should lock the instance using the lock instance variable for the duration of the operation, and unlock it afterwards.

Application code must not call this method directly, it is for internal use only.

This method is only for the use of the -insertBLOBs:intoStatement:length:withMarker:length:giving: method.
Subclasses which need to insert binary data into a statement must implement this method to copy the escaped data into place and return the number of bytes actually copied.

This method is a convenience method provided for subclasses which need to insert escaped binary data into an SQL statement before sending the statement to a backend server process. This method makes use of the -copyEscapedBLOB:into: and -lengthOfEscapedBLOB: methods, which must be implemented by the subclass.

The blobs array is an array containing the original SQL statement string (unused by this method) followed by the data items to be inserted.

The statement and sLength arguments specify the datastream to be copied and into which the BLOBs are to be inserted.

The marker and mLength arguments specify the sequence of marker bytes in the statement which indicate a position for insertion of an escaped BLOB.

The method returns either the original statement or a copy containing the escaped BLOBs. The length of the returned data is stored in result.

This method is only for the use of the -insertBLOBs:intoStatement:length:withMarker:length:giving: method.
Subclasses which need to insert binary data into a statement must implement this method to return the length of the escaped bytestream which will be inserted.