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In SQLite, how do I select items in one table that are not in another?

There are two tables in SQLite:

Table1:
-------
id
name

Table2:
-------
id
temp_name

My question is, how do I write a SQL query that returns names in Table2

that are not in Table1

?

For example:

Table1:
-------
1, 'john'
2, 'boda',
3, 'cydo',
4, 'linus'

Table2:
-------
1123, 'boda'
2992, 'andy',
9331, 'sille',
2,    'cydo'

In this example, the SQL query should return items to andy

and sille

from Table2

because they are not in Table1

.

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5 answers


This is how it is done in "obvious" standard SQL:

select *
from table2
where temp_name not in (select name from table1)


There are other methods such as use left outer join

, exists

in sentence where

and operation except

.

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select name
from table2
except
select temp_name
from table1


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Left side attachment version:

select t2.* from Table2 t2
left outer join Table1 t1 on t1.name = t2.temp_name
where t2.temp_name is null
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Method EXISTS

mentioned by Gordon:

SELECT *
FROM Table2
WHERE NOT EXISTS (SELECT 1
                  FROM Table1
                  WHERE Table1.name = Table2.temp_name)
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I was curious to know which of the options works best for my use case, and thought it might help others if I shared my results:

In short, pick where not at the fastest (for me!). It's also worth noting that using all options will return duplicates except for exceptions.

Option 0: SELECT {c2} FROM {t2} WHERE {c2} not in (SELECT {c1} FROM {t1});
    Entries returned: 1098, Unique entries returned: 357
    Average: 1.8680 seconds
    Entries returned: 0, Unique entries returned: 0
    Average: 0.6664 seconds
Option 1: SELECT {c2} FROM {t2} EXCEPT SELECT {c1} FROM {t1};
    Entries returned: 357, Unique entries returned: 357
    Average: 3.9455 seconds
    Entries returned: 0, Unique entries returned: 0
    Average: 3.3074 seconds
Option 2: SELECT {t2}.{c2} FROM {t2} LEFT OUTER JOIN {t1} ON {t1}.{c1} = {t2}.{c2} WHERE {t1}.{c1} IS null;
    Entries returned: 1098, Unique entries returned: 357
    Average: 2.3330 seconds
    Entries returned: 0, Unique entries returned: 0
    Average: 1.1982 seconds
Option 3: SELECT {c2} FROM {t2} WHERE NOT EXISTS (SELECT 1 FROM {t1} WHERE {c1} = {t2}.{c2});
    Entries returned: 1098, Unique entries returned: 357
    Average: 2.6945 seconds
    Entries returned: 0, Unique entries returned: 0
    Average: 0.9737 seconds

Here's the code I used to run the numbers:

import sqlite3
import timeit

# Database path here
database = "database.db"

# Your table and column names here
t1, c1 = 'Table_1', 'name'
t2, c2 = 'Table_2', 'temp_name'

# Reverse the test
dbs = [{'t1': t1, 'c1':c1, 't2': t2, 'c2': c2},
       {'t1': t2, 'c1':c2, 't2': t1, 'c2': c1}]

commands = ["SELECT {c2} FROM {t2} WHERE {c2} not in (SELECT {c1} FROM {t1});",
"SELECT {c2} FROM {t2} EXCEPT SELECT {c1} FROM {t1};",
"SELECT {c2} FROM {t2} LEFT OUTER JOIN {t1} ON {t1}.{c1} = {t2}.{c2} WHERE {t2}.{c2} IS null;",
"SELECT {c2} FROM {t2} WHERE NOT EXISTS (SELECT 1 FROM {t1} WHERE {c1} = {t2}.{c2});",]

for i, c in enumerate(commands):
    print("Option {}: {}".format(i, c))
    for db in dbs:
        co = c.format(**db)
        foo = sqlite3.connect(database).execute(co).fetchall()
        # Sanity check that entries have been found and how many
        print("\tEntries returned: {}, Unique entries returned: {}".format(len(foo), len({a[0] for a in foo})))
        # Reconnect to the database each time - I can't remember if there any caching
        t = timeit.repeat(lambda: sqlite3.connect(database).execute(co).fetchall(), repeat=5, number=1)
        print('\tAverage: {:.4f} seconds'.format(statistics.mean(t)))
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