LEFT JOIN仅显示第一张表的第一行

Question

I have two:

Firest table: table1

id   |  text   
-------------
1       t1      
2       t2           
...

Second table: table2

id   |  idTable1 |  text2
-------------------------------
1       1           text1Table2
2       1           text2Table2
3       1           text3Table2
4       2           text4Table2
5       2           text5Table2
...

If i do LEFT JOIN:

SELECT * FROM table1 t1 LEFT JOIN table2 t2 ON t1.id=t2.idTable1

the result is this:

id  |   text |  id | idTable1 | text2
------------------------------------------
1       t1      1   1           text1Table2
1       t1      2   1           text2Table2
1       t1      3   1           text3Table2
2       t2      4   2           text4Table2
2       t2      5   2           text5Table2
...

But i want the rows that are duplicated in the first table to be displayed only once like this:

id  |   text |  id | idTable1 | text2
------------------------------------------
1       t1      1   1           text1Table2
1       -       2   1           text2Table2
1       -       3   1           text3Table2
2       t2      4   2           text4Table2
2       -       5   2           text5Table2
...

Edit: And this result to add in mysql view

Answer 1

You can do this using variables:

SELECT t1.id,
       if(@prevtext = text, '-', if(@prevtext := text, t1.text, t1.text)) as text,
       t2.id, t2.idTable1, t2.text2
FROM table1 t1 LEFT JOIN
     table2 t2
     ON t1.id = t2.idTable1 CROSS JOIN
     (select @prevtext := '') vars
ORDER BY t1.id, t1.text;

However, I discourage you from doing this. A result set in SQL has the same properties as a table. Namely, each row is independent of the other rows. When you put data in this format, you are requiring a particular ordering of the result set, so it no longer has the properties of a SQL table.

EDIT:

I thought this was impossible to do with a subquery compatible with a view, but it may be possible given the data your query (where the t2 id is ordered):

SELECT t1.id,
       (case when text = (select text
                          from table2 tt2
                          where tt2.id < t2.id
                          order by tt2.id desc
                          limit 1
                         )
             then '-' else text
        end) as text
       t2.id, t2.idTable1, t2.text2
FROM table1 t1 LEFT JOIN
     table2 t2
     ON t1.id = t2.idTable1 
ORDER BY t1.id, t2.id;

Answer 2

You can use a case statement and a subquery to determine whether a row is the first (ie has the lowest table2.id ) for a given table2.idTable

CREATE VIEW myView AS SELECT 
    t1.id id1,
    (CASE
        WHEN t2.idTable1 IS NULL OR NOT EXISTS (
            SELECT 1 
            FROM table2 t3
            WHERE t3.idTable1 = t2.idTable1
            AND t3.id < t2.id
        ) THEN t1.text ELSE '-'
    END) text1,
    t2.id id2,    
    t2.idTable1,
    t2.text2
FROM table1 t1 
LEFT JOIN table2 t2 ON t1.id = t2.idTable1