如果找不到匹配项，如何附加列数据？

Question

I have table1 and table2 as following

Table1

first_name | last_name | t1_id

Table2

T2_id | color | length

Table1 may have an id value which doesn't exist in Table2 id, however it may exist sometimes. I want to write a query that gives the following result: First_name | last_name | t1_id | color | length Where in case if the id value doesn't exist in table2 the result should be like:

First_name | last_name | t1_id | color | length
-----------------------------------------------
Johan      | Mike      | 33    |   -   |   -

My try:

SELECT first_name,
       last_name,
       t1_id,
       color,
       length
FROM Table1, Table2
WHERE t1_id = t2_id

But this show the result when the value of t1_id only exists in t2_id .

Answer 1

Use LEFT join to get all rows from first table even if their associations are not present in second table

SELECT first_name,
       last_name,
       t1_id,
       color,
       length
FROM Table1
LEFT JOIN Table2 ON t1_id = t2_id

You could use COALESCE(column,'-') to show - if column is null

SELECT first_name,
       last_name,
       t1_id,
       COALESCE(color,'-'),
       COALESCE(length,'-')
FROM Table1
LEFT JOIN Table2 ON t1_id = t2_id

Answer 2

Use Left Join

SELECT first_name,
       last_name,
       t1_id,
       color,
       length
FROM Table1
LEFT JOIN Table2 ON t1_id = t2_id

Answer 3

you can use mysql left join join in this case if right table data not found then null

SELECT t1.first_name,
       t1.last_name,
       t1.t1_id,
       t2.color,
       t2.length
FROM table1  t1
LEFT JOIN table2 t2 
ON t1.t1_id = t2.t2_id;

in this sql i use alias but if you do not want it then

SELECT first_name,
       last_name,
       t1_id,
       color,
       length
    FROM table1  
    LEFT JOIN table2  
    ON t1_id = t2_id;

for more information

https://dev.mysql.com/doc/refman/5.7/en/left-join-optimization.html

Answer 4

You should make use of LEFT JOIN

SELECT *
FROM Table1 LEFT JOIN Table2
ON t1_id = t2_id

Answer 5

Try this one :

SELECT first_name,
       last_name,
       t1_id,
       color,
       length
FROM Table1, Table2
WHERE t1_id = t2_id(+)

Regards, Damodar