The columns obect command returns a list of the names of the columns in the result set. The columns will appear in the same order as they appeared in the SQL statement that performed the database query. If the SQL statement does not return a set of columns (for instance, if it is an INSERT, UPDATE, or DELETE statement), the columns command will return an empty list.
The rowcount object command returns the number of rows in the database that were affected by the execution of an INSERT, UPDATE or DELETE statement. For a SELECT statement, the row count is unspecified.
The nextlist object command sets the variable given by varname in the caller's scope to the next row of the results, expressed as a list of column values. NULL values are replaced by empty strings. The columns of the result row appear in the same order in which they appeared on the SELECT statement. The return of nextlist is 1 if the operation succeeded, and 0 if the end of the result set was reached.
The nextdict object command sets the variable given by varname in the caller's scope to the next row of the results, expressed as a dictionary. The dictionary's keys are column names, and the values are the values of those columns in the row. If a column's value in the row is NULL, its key is omitted from the dictionary. The keys appear in the dictionary in the same order in which the columns appeared on the SELECT statement. The return of nextdict is 1 if the operation succeeded, and 0 if the end of the result set was reached.
The nextrow object command is precisely equivalent to the nextdict or nextlist object command, depending on whether -as dicts (the default) or -as lists is specified.
Some databases support the idea of a single statement that returns multiple sets of results. The nextresults object command is executed, typically after the nextlist of nextdict object command has returned 0, to advance to the next result set. It returns 1 if there is another result set to process, and 0 if the result set just processed was the last. After calling nextresults and getting the return value of 1, the caller may once again call columns to get the column descriptions of the next result set, and then return to calling nextdict or nextlist to process the rows of the next result set. It is an error to call columns, nextdict, nextlist or nextrow after nextresults has returned 0.
The allrows object command sets the variable designated by the -columnsvariable option (if present) to the result of the columns object command. It then executes the nextrow object command repeatedly until the end of the result set is reached. If nextresults returns a nonzero value, it executes the above two steps (columns followed by iterated nextrow calls) as long as further results are available. The rows returned by nextrow are assembled into a Tcl list and become the return value of the allrows command; the last value returned from columns is what the application will see in -columnsvariable.
The foreach object command sets the variable designated by the -columnsvariable option (if present) to the result of the columns object command. It then executes the nextrow object command repeatedly until the end of the result set is reached, storing the successive rows in the variable designated by varName. For each row, it executes the given script. If the script terminates with an error, the error is reported by the foreach command, and iteration stops. If the script performs a break operation, the iteration terminates prematurely. If the script performs a continue operation, the iteration recommences with the next row. If the script performs a return, results are the same as if a script outside the control of foreach had returned. Any other unusual return code terminates the iteration and is reported from the foreach.
Once nextrow returns 0, the foreach object command tries to advance to the next result set using nextresults. If nextresults returns 1, the above steps (columns and nextrow, with script invocation) are repeated as long as more result sets remain. The script will always see the correct description of the columns of the current result set in the variable designated byt -columnsvariable. At the end of the call, the variable designated by -columnsvariable will have the description of the columns of the last result set.
The close object command deletes the result set and frees any associated system resources.
Copyright © 2008 by Kevin B. Kenny.