MySQL从其他表插入一个表列

Question

I need to copy two columns from two different tables into new table:

t1: col1  col2       t2: col3  col4
    a1     b1              c1   d1
    a2     b2              c2   d2
                           c3   d3

I need to get this:

t3: col1  col4
     a1    d1
     a2    d2
     null  d3

I used query:

INSERT INTO t3 (col1,col4) SELECT t1.col1, t2.col4 FROM t1,t2

but i am getting this:

t3: col1  col4
     a1    d1
     a2    d1
     a1    d2
     a2    d2
     a1    d3
     a2    d3

Can anyone help? Should I use some other query? Tnx!

Answer 1

You are using a CROSS JOIN , or carthesian product. This means that all rows in the left table will match all rows in the right table. This happens because you didn't tell MySQL how the tables rows should match.

You need to add the JOIN keyword and specify an ON clause, which specifies the matching rule - which values must be equal. For example:

SELECT a.col1, b.col4 FROM t1 a INNER JOIN t2 b ON a.id_t2 = b.id;

Answer 2

To be able to retrieve the above result, you'll need to use the left outer join , but however I can't find any linking columns.

 between your two tables;

INSERT INTO t3 (t1.col1,t2.col4) 
SELECT t1.col1, t2.col4 
FROM t2
Left Outer Join t1
on --> Your link column between the tables

Answer 3

It appears that your simply attempting to join the first row in one table to the first row in another table, then insert them into a third table. This SQL should perform the inserts:

insert into table3
  select col1, col4 from (select @rownum2 := @rownum2+1 as rown, col4 from table2
     JOIN (SELECT @rownum2 := 0) r2) t2
  left outer join
    (
    select @rownum := @rownum+1 as rown, col1 from table1
    JOIN (SELECT @rownum := 0) r) t1 
  on t2.rown = t1.rown;

SQL Fiddle: http://sqlfiddle.com/#!2/f9314f