MySQL查询：涉及2个表

Question

I have two tables

table 1 : id, name, age, gender.
table 2(friend) : id1,id2. (stored only once i.e only one entry for a unique pair)

I need to write a query that takes an id and then return the result set containing the rows from table 1 corresponding to all his/hers friend.

I tried this Query

"SELECT * FROM table1 WHERE id IN ("
+ "(SELECT id2 FROM table2 WHERE (id1="+given_id+")) UNION "
+ "(SELECT id1 FROM table2 WHERE (id2="+given_id+")));"

but it does'nt seem to work says syntax error ... could anyone help me with this query.

Efforts appreciated ... Thanks a lot

Answer 1

You need to single quote given_id , because otherwise MySQL will think it is a column name.
This code will also work if given_id is a number.

"SELECT t1.* "
+"FROM table1 t1 "
+"INNER JOIN table2 t2 ON (t1.id = t2.id1 OR t1.id = t2.id2) "
+"WHERE '"+given_id+"' IN (t2.id1, t1.id) "

Answer 2

Please use a join query:

"SELECT `table1`.* FROM (`table1`) 
    LEFT OUTER JOIN `table2` ON 
        `table1`.`id` = `table2`.`id1`
    WHERE `table2`.`id2` =
    " + given_id;

Answer 3

You can narrow down your results by restricting conditions on a join. Since you will have to join your table2 on more than one id, you will have to join to it twice and alias the table names.

I believe something like this may do the trick:

SELECT table1.*
FROM table1
JOIN table2 t2 ON table1.id = t2.id2
  AND t2.id1 = {given_id}
JOIN table2 t3 ON table1.id = t3.id1
  AND t3.id2 = {given_id};

Answer 4

您可以执行类似的操作。

SELECT * FROM table1 t1, table2 t2, WHERE t1.id = t2.id2