Question

我需要比较SQLlite数据库（.db）中的两列，如果我发现巧合，则修改第三列。即：

 A B C 
-------
11 1  F
21 3  F
54 21 V
65 4  F
96 9  F
82 5  F
71 11 V
53 5  F

如果任何数量的列＆＃34; B＆＃34;列在＆＃34; A＆＃34;列中的结果＆＃34; C＆＃34;将是＆＃34; V＆＃34;。

专栏＆＃34; C＆＃34;默认情况下，在开始比较之前，将为所有行设置为F.我使用的是Python 2.7和SQLlite 3，但我真的不知道如何进行这种比较。

Answer 1

UPDATE mytable 
    SET C = 'V' 
WHERE B IN (SELECT A FROM mytable)

这是一个完整的Python程序，用于显示它的工作原理：

# from __future__ import print_function # uncomment import for Python 2.7
import sqlite3

connection = sqlite3.connect(':memory:')
cursor = connection.cursor()
cursor.execute('CREATE TABLE mytable (A INTEGER, B INTEGER, C TEXT)')

initial_AB_vals = [(11, 1), (21, 3), (54, 21), (65, 4), (96, 9), (82, 5), (71, 11), (53, 5)]
cursor.executemany("INSERT INTO mytable(A, B, C) VALUES (?, ?, 'F')", initial_AB_vals)

print('BEFORE UPDATE')
for row in cursor.execute('SELECT * FROM mytable'):
    print('    {:2d} {:2d} {}'.format(*row))

cursor.execute("UPDATE mytable SET C = 'V' WHERE B IN (SELECT A FROM mytable)") 

print()
print('AFTER UPDATE')
for row in cursor.execute('SELECT * FROM mytable'):
    print('    {:2d} {:2d} {}'.format(*row)) 

connection.commit()
connection.close()

输出

BEFORE UPDATE
    11  1 F
    21  3 F
    54 21 F
    65  4 F
    96  9 F
    82  5 F
    71 11 F
    53  5 F

AFTER UPDATE
    11  1 F
    21  3 F
    54 21 V
    65  4 F
    96  9 F
    82  5 F
    71 11 V
    53  5 F

Answer 2

您可以使用UPDATE ... WHERE EXISTS更新您的值，如：

 UPDATE mt
    SET mt.C = 'V'
   FROM mytable mt
  WHERE EXISTS (SELECT 1 FROM mytable mt2 WHERE mt.A = mt2.B)
;

列比较SQL和Python

2 个答案: