The select() method is used to retrieve information from a database and looks like this:
Falsethe function returns the SQL string to perform the desired operations. If
Truethe SQL is executed and the results converted and returned in the appropriate form. If not specified takes the value specifed in the cursor which by default is
'dict'to return them as a tuple of dictionary objects,
'tuple'to return them as a tuple of tuples or
'dtuple'to return them as a tuple of dtuple objects which can be treated as a tuple or a dictionary. If not specified takes the value specifed in the cursor which by default is
To select some information from a database using an SQL string you would use the following command:
SELECT column_name(s) FROM table_name
For example consider the table below:
# Table Person +----------+-----------+---------+-------------+ | LastName | FirstName | Address | DateOfBirth | +----------+-----------+---------+-------------+ | Smith | John | Bedford | 1980-01-01 | +----------+-----------+---------+-------------+ | Doe | John | Oxford | 1981-12-25 | +----------+-----------+---------+-------------+
To retrieve a list of the surnames and dates of birth of all the people in the table you would use the following code:
rows = cursor.select(['LastName', 'DateOfBirth'], 'Person')
The result from this call would be a tuple of TupleDescriptor objects which can be treated as a tuple or a dictionary:
>>> for record in rows: ... print record, record ... print record['LastName'], record['DateOfBirth'] ... Smith 1980-01-01 Smith 1980-01-01 Doe 1981-12-25 Doe 1981-12-25
Using the select() method, information you select from a field is automatically converted to the correct Python type. Integer fields return Integers, Date fields return
If you are only selecting one field you can just specify the fieldname as a string
'LastName' rather than
The example above selected every
DateOfBirth field from the table. To limit the information selected you need to specify the
where parameter in the same way you would for any SQL query.
>>> rows=cursor.select('LastName','Person',where="LastName='Smith'") >>> for record in rows: ... print record['LastName'], record['DateOfBirth'] ... 'Smith'
we had to specify the value
Smith as properly encoded SQL. See the section Encoders and Decoders for information on how to encode these strings automatically.
More complex expressions can be built into where clauses. See a guide on SQL such as the one at www.w3schools.com for more information.
You can specify the order in which the results are sorted using the
order parameter. It is used as follows:
>>> for record in cursor.select('LastName', 'Person', order="'LastName'"): ... print record['LastName'] ... 'Doe' 'Smith' >>> for record in cursor.select('LastName', 'Person', order="LastName DESC"): ... print record['LastName'] ... 'Smith' 'Doe'
DESCafter the column to order by, the order is reversed.
You can place a number of Columns after each other. For example
order="LastName DESC DateOfBirth" could be used to order the results in decending order by
LastName and if any results have the same last name, order them by
If you want to specify any other SQL on the end of the statement you can place it in the
rest fields take user-formatted strings as input. It is still possible that the SQL commands contained in these strings will have different results on different databases so please be careful with the SQL you use.
If you do not want the SQL to actually be executed you can set the
autoExecute parameter of the select() method to
False. You can then manually execute it using
>>> sql = cursor.select(['LastName', 'DateOfBirth'], 'Person', autoExecute=False) >>> sql 'SELECT LastName, DateOfBirth FROM Person' >>> cursor.execute(sql) >>> cursor.fetchall() (('Smith','1980-01-01'),('Doe','1981-12-25'))
Many SQL databases support the use of aliases. An alias is when you select certain data from a table under a different name. The SQL you would use might be:
SELECT LastName AS Surname FROM Person
With the select() method this would be:
>>> rows = cursor.select('LastName', 'Person', as='Surname') >>> rows['Surname'] 'Smith'
In this example we accessed the value of the
LastName column as if it was called
To specify more than one alias you specify the columns as a list or tuple and specify the aliases as a list or tuple with the same number of aliases in the same order. If you don't want to specify aliases for every coulmn use None in place of the alias name.
>>> rows = cursor.select(['LastName','Address','DateOfBirth'], 'Person', as=['Surname', None, 'Birth']) >>> rows['Surname'] 'Smith' >>> rows['Address'] 'Bedford' >>> rows['Birth'] '1980-01-01'
The select() allows you to select information from multiple tables. In order to do this you must specify the tables you wish to select from as a list or tuple and use the fully qualified column name for each table you want to column you want to select from.
rows = cursor.select( ['table1.LastName', 'table2.Surname'], ['table1','table2'], where="table1.Surname = table2.Surname" ) print rows['table2.Surname'] 'Smith'
You can also combine joins with aliases by specifying each column as a tuple
('tableName.columName', 'alias') as described in that section above on aliases.
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