MySQL的左外连接排除

Question

I have 2 tables ' conta ' and ' details ' and both the tables have null values and data in different case

conta
id  col1  col2  col3  col4
1  Hi  Bye  See YOU  
2  Hello  (NULL)  (NULL)  (NULL)

details

id  new_column1  new_column2  new_column3
1  bye  see  you

I want to apply join based on col2=new_column1 and col3 = new_column2 and col4 = new_column3 and get the values that are present in conta and not in details , so my output will be

conta
id  col1  col2  col3  col4
2  hello  (NULL)  (NULL)  (NULL)

But somehow i am unable to do so. I wrote below query, but its simply not resulting me the values i want.

SELECT `id`,`col1`,`col2`,`col3`,`col4` FROM `conta`
WHERE LOWER(`col2`) + LOWER(`col3`) + LOWER(`col4`) NOT IN (SELECT DISTINCT(LOWER(`new_column1`) + LOWER(`new_column2`) + LOWER(`new_column3`))
 FROM `details`);

It simply give me no results! in the display

Any help?

Edit: I tried below query as suggested by @Uueerdo and it isn't giving me what i want.

SELECT conta.id,`col1`,`col2`,`col3`,`col4` FROM `conta`
LEFT OUTER JOIN `details`
ON ((conta.col2 IS NULL AND details.new_column1 IS NULL) 
    OR (LOWER(conta.col2) = LOWER(details.new_column1)))
AND ((conta.col3 IS NULL AND details.new_column2 IS NULL) 
    OR (LOWER(conta.col3) = LOWER(details.new_column2)))
AND ((conta.col4 IS NULL AND details.new_column3 IS NULL) 
    OR (LOWER(conta.col4) = LOWER(details.new_column3)))
WHERE details.id IS NULL

In the output in col2 i see a value ' Operations ' which is also present in new_column1 in details table. This means it shouldn't be present in the output as i am trying to apply left outer join exclude I even tried using LEFT JOIN instead of LEFT OUTER JOIN and it isn't working either

Edit2 : I found the solution. the query works and does the job. Exept that i had to run a command to replace all blank cells in the columns where i am applying join to NULL values.

Answer 1

You're better off using a SELECT .... FROM a LEFT JOIN b ON conditions WHERE b.id IS NULL style of query; null comparisons are a little different (and can be handled join conditions).

For example these evaluate to NULL, which is not true, which is false:

• NULL = NULL
• NULL IN (NULL)

But you can do things like this to compare nulls more easily:

• ISNULL(a, x) = ISNULL(b, x)
• (a IS NULL AND b IS NULL)

So you're join condition can be something like:

[...]
ON ((conta.col2 IS NULL AND details.new_column1 IS NULL) 
    OR (LOWER(conta.col2) = LOWER(details.new_column1)))
AND ((conta.col3 IS NULL AND details.new_column2 IS NULL) 
    OR (LOWER(conta.col3) = LOWER(details.new_column2)))
[and so on...]
WHERE details.id IS NULL

This assumes details has some sort of non-null row identification field that can used to reliably determine if there was a match or not.

---

Edit: The precise problem with your current query (aside from the null issues I previously outlined) is that + is not concatenation in MySQL, it is addition. With the data you've shown both LOWER(col2) + LOWER(col3) + LOWER(col4) and LOWER(new_column1) + LOWER(new_column2) + LOWER(new_column3) with yield 0 for the rows without NULL values in them. You would need to use the CONCAT() function to do the operation instead; but I'd discourage it because CONCAT('abc', 'def', '') is equal to CONCAT('ab', 'cd', 'ef') .

Sidenote: DISTINCT is not a function, the () will have no effect ( other than that they would cause a problem if they contained more than one result field ).

---

You can keep your general format, and the aforementioned null issues, by a simple change with this format : WHERE (a, b, c) IN (SELECT a, b, c FROM ....

Answer 2

You can do a left join, then test for a null in the 2nd table to find the rows in the first table that didn't match any rows in the 2nd table.

SELECT 
    a.`id`,
    a.`col1`,
    a.`col2`,
    a.`col3`,
    a.`col4` 
FROM `conta` a
LEFT JOIN `details` b
ON a.`col2` like b.`new_column1`
    AND a.`col3` like b.`new_column2`
    AND a.`col4` like b.`new_column3`
WHERE b.`id` IS NULL

Answer 3

You could use the EXISTS operator to create an anti-semi join. Please take a look at this link: https://www.techonthenet.com/mysql/exists.php

Example Query:

SELECT
    id,
    col1,
    col2,
    col3,
    col4
FROM conta
WHERE NOT EXISTS (
    SELECT 1
    FROM details
    WHERE conta.col2 LIKE details.new_column1
        AND conta.col3 LIKE details.new_column2
        AND conta.col4 LIKE details.new_column3
)

Answer 4

Your Problem Solution Is inner join your (contact table) with (details table) and get (contact.*) All columns where Contact.col1 !=details.new_column1

here is a query

Select conta.* from conta inner join details on conta.col1!=details.new_column1

You Can And More Where column in inner join