MYSQL：从其他表插入数据

Question

I'm trying to insert data from two other tables. I currently have two tables, t1 and t2:

t1:

col1  col2  col3  col4  col5
 A     A     B     A     C
 B     B     C     C     A
 B     C     B     B     A
 C     A     A     A     B

and t2:

         A   B   C
col1     4   99  81
col2     50  26  38
col3     36  38  11
col4     16  49  70
col5     42  83  93

My goal is to form a new table which would simply use data from t1 and t2 and form a new table such as:

col1  col2  col3  col4  col5
 4     50    38    16    93
 99    26    11    70    42
 99    38    38    49    42
 81    50    36    16    83

I have been using CASE-function, but I've faced problems with attaching the tables. Any advices?

Thanks!

Answer 1

If we convert t2 from a wide table to a long table:

mysql> CREATE VIEW tidyt2 AS
       SELECT x as 'col', 'A' as 'label', A 'value' FROM t2
       UNION SELECT x as 'col', 'B' as 'label', B 'value' FROM t2
       UNION SELECT x as 'col', 'C' as 'label', C 'value' FROM t2;

Query OK, 0 rows affected (0.03 sec)

mysql> select * from tidyt2;
+------+-------+-------+
| col  | label | value |
+------+-------+-------+
| col1 | A     |     4 |
| col2 | A     |    50 |
| col3 | A     |    36 |
| col4 | A     |    16 |
| col5 | A     |    42 |
| col1 | B     |    99 |
| col2 | B     |    26 |
| col3 | B     |    38 |
| col4 | B     |    49 |
| col5 | B     |    83 |
| col1 | C     |    81 |
| col2 | C     |    38 |
| col3 | C     |    11 |
| col4 | C     |    70 |
| col5 | C     |    93 |
+------+-------+-------+
15 rows in set (0.00 sec)

Then the desired table can be expressed using left joins:

mysql> SELECT t21.value as 'col1'
              , t22.value as 'col2'
              , t23.value as 'col3'
              , t24.value as 'col4'
              , t25.value as 'col5'
         FROM t1 
         LEFT JOIN tidyt2 as t21 ON t1.col1 = t21.label AND t21.col='col1'
         LEFT JOIN tidyt2 as t22 ON t1.col2 = t22.label AND t22.col='col2'
         LEFT JOIN tidyt2 as t23 ON t1.col3 = t23.label AND t23.col='col3'
         LEFT JOIN tidyt2 as t24 ON t1.col4 = t24.label AND t24.col='col4'
         LEFT JOIN tidyt2 as t25 ON t1.col5 = t25.label AND t25.col='col5';

+------+------+------+------+------+
| col1 | col2 | col3 | col4 | col5 |
+------+------+------+------+------+
|    4 |   50 |   38 |   16 |   93 |
|   99 |   26 |   11 |   70 |   42 |
|   99 |   38 |   38 |   49 |   42 |
|   81 |   50 |   36 |   16 |   83 |
+------+------+------+------+------+
4 rows in set (0.00 sec)