How to get the numbers of data row from sqlite3 da

问题:

I am trying to get the numbers of rows returned from an sqlite3 database in python but it seems the feature isn't available:

Think of php mysqli_num_rows() in mysql

Although I devised a means but it is a awkward: assuming a class execute sql and give me the results:

# Query Execution returning a result
data = sql.sqlExec("select * from user")
# run another query for number of row checking, not very good workaround
dataCopy = sql.sqlExec("select * from user")
# Try to cast dataCopy to list and get the length, I did this because i notice as soon 
# as I perform any action of the data, data becomes null
# This is not too good as someone else can perform another transaction on the database 
# In the nick of time
    if len(list(dataCopy)) :
        for m in data :
            print("Name = {}, Password = {}".format(m["username"], m["password"]));
    else :
        print("Query return nothing")

Is there a function or property that can do this without stress.

回答1:

Normally, cursor.rowcount would give you the number of results of a query.

However, for SQLite, that property is often set to -1 due to the nature of how SQLite produces results. Short of a COUNT() query first you often won't know the number of results returned.

This is because SQLite produces rows as it finds them in the database, and won't itself know how many rows are produced until the end of the database is reached.

From the documentation of cursor.rowcount:

Although the Cursor class of the sqlite3 module implements this attribute, the database engine’s own support for the determination of “rows affected”/”rows selected” is quirky.

For executemany() statements, the number of modifications are summed up into rowcount.

As required by the Python DB API Spec, the rowcount attribute “is -1 in case no executeXX() has been performed on the cursor or the rowcount of the last operation is not determinable by the interface”. This includes SELECT statements because we cannot determine the number of rows a query produced until all rows were fetched.

Emphasis mine.

For your specific query, you can add a sub-select to add a column:

data = sql.sqlExec("select (select count() from user) as count, * from user")

This is not all that efficient for large tables, however.

If all you need is one row, use cursor.fetchone() instead:

cursor.execute('SELECT * FROM user WHERE userid=?', (userid,))
row = cursor.fetchone()
if row is None:
    raise ValueError('No such user found')

result = "Name = {}, Password = {}".format(row["username"], row["password"])

回答2:

Use following:

dataCopy = sql.sqlExec("select count(*) from user")
values = dataCopy.fetchone()
print values[0]

回答3:

I've found the select statement with count() to be slow on a very large DB. Moreover, using fetch all() can be very memory-intensive.

Unless you explicitly design your database so that it does not have a rowid, you can always try a quick solution

cur.execute("SELECT max(rowid) from Table")
n = cur.fetchone()[0]

This will tell you how many rows your database has.

回答4:

import sqlite3
conn = sqlite3.connect(path/to/db)
cursor = conn.cursor()
cursor.execute("select * from user")
results = cursor.fetchall()
print len(results)

len(results) is just what you want

回答5:

A simple alternative approach here is to use fetchall to pull a column into a python list, then count the length of the list. I don't know if this is pythonic or especially efficient but it seems to work:

rowlist = []
c.execute("SELECT {rowid} from {whichTable}".\
          format (rowid = "rowid", whichTable = whichTable))
rowlist = c.fetchall ()
rowlistcount = len(rowlist)
print (rowlistcount)

回答6:

this code worked for me:

import sqlite3
con = sqlite3.connect(your_db_file)
cursor = con.cursor()
result = cursor.execute("select count(*) from your_table") #returns array of tupples
num_of_rows = result[0][0]