为什么选择0，而不是选择-WhySELECT0,…insteadofSELECT

Lets say I have a SQLite database that contains a table:

假设我有一个SQLite数据库，其中包含一个表:

sqlite> create table person (id integer, firstname varchar, lastname varchar);

Now I want to get every entry which is in the table.

现在我想要得到表格中的每一个元素。

sqlite> select t0.id, t0.firstname, t0.lastname from person t0;

This works fine and this it what I would use. However I have worked with a framework from Apple (Core Data) that generates SQL. This framework generates a slightly different SQL query:

这很好，这就是我要用的。然而，我使用了一个来自苹果的框架(核心数据)来生成SQL。这个框架生成的SQL查询稍有不同:

sqlite> select 0, t0.id, t0.firstname, t0.lastname from person t0;

Every SQL query generated by this framework begins with "select 0,". Why is that?

这个框架生成的每个SQL查询都以“select 0”开头。这是为什么呢?

I tried to use the explain command to see whats going on but this was inconclusive - at least to me.

我试着使用explain命令来查看发生了什么，但这是不确定的——至少对我来说是这样。

sqlite> explain select t0.id, t0.firstname, t0.lastname from person t0;
addr        opcode      p1          p2          p3          p4          p5          comment   
----------  ----------  ----------  ----------  ----------  ----------  ----------  ----------
0           Trace       0           0           0                       00          NULL      
1           Goto        0           11          0                       00          NULL      
2           OpenRead    0           2           0           3           00          NULL      
3           Rewind      0           9           0                       00          NULL      
4           Column      0           0           1                       00          NULL      
5           Column      0           1           2                       00          NULL      
6           Column      0           2           3                       00          NULL      
7           ResultRow   1           3           0                       00          NULL      
8           Next        0           4           0                       01          NULL      
9           Close       0           0           0                       00          NULL      
10          Halt        0           0           0                       00          NULL      
11          Transactio  0           0           0                       00          NULL      
12          VerifyCook  0           1           0                       00          NULL      
13          TableLock   0           2           0           person      00          NULL      
14          Goto        0           2           0                       00          NULL 

And the table for the second query looks like this:

第二个查询的表格是这样的:

sqlite> explain select 0, t0.id, t0.firstname, t0.lastname from person t0;
addr        opcode      p1          p2          p3          p4          p5          comment   
----------  ----------  ----------  ----------  ----------  ----------  ----------  ----------
0           Trace       0           0           0                       00          NULL      
1           Goto        0           12          0                       00          NULL      
2           OpenRead    0           2           0           3           00          NULL      
3           Rewind      0           10          0                       00          NULL      
4           Integer     0           1           0                       00          NULL      
5           Column      0           0           2                       00          NULL      
6           Column      0           1           3                       00          NULL      
7           Column      0           2           4                       00          NULL      
8           ResultRow   1           4           0                       00          NULL      
9           Next        0           4           0                       01          NULL      
10          Close       0           0           0                       00          NULL      
11          Halt        0           0           0                       00          NULL      
12          Transactio  0           0           0                       00          NULL      
13          VerifyCook  0           1           0                       00          NULL      
14          TableLock   0           2           0           person      00          NULL      
15          Goto        0           2           0                       00          NULL     

3 个解决方案

  A      B
+---+  +---+------+
| a |  | a | b    |
+---+  +---+------+
| 0 |  | 0 |    1 |
+---+  +---+------+
| 1 |  | 1 | NULL |
+---+  +---+------+
| 2 |
+---+

SELECT A.a, B.b
FROM A
LEFT JOIN B
ON B.a = A.a

  Results
+---+------+
| a | b    |
+---+------+
| 0 |    1 |
+---+------+
| 1 | NULL |
+---+------+
| 2 | NULL |
+---+------+

SELECT A.a, B.x, B.b
FROM A
LEFT JOIN (SELECT 0 AS x, B.a, B.b FROM B) AS B
ON B.a = A.a

  Results
+---+------+------+
| a | x    | b    |
+---+------+------+
| 0 |    0 |    1 |
+---+------+------+
| 1 |    0 | NULL |
+---+------+------+
| 2 | NULL | NULL |
+---+------+------+

WHERE 1 = 1

AND x = y

String query = "SELECT 0";

for (Column col in columnList)
    query += ", col";