Ok so I know we can join tables 'across' in MySQL to tables using a id column.
I have two databases where I need to join two tables that are almost identical.
I need to join these 'down', e.g similar to inserting the whole of one table into the other. Not a normal cross join.
Does MySQL allow me to do this?
UNION is a good way to accomplish this.
SELECT * FROM table1
UNION
SELECT * FROM table2
Just make sure number of columns being returned are the same, and data type matches.
Yes, You can use UNION - the only condition is to select the same amount of columns from both select quesries (both tables):
SELECT a, b, c, d FROM table1
UNION
SELECT d, e, f, g FROM table2
Common issue is to order the results from UNION, so here is solution:
SELECT a AS order_by, b, c, d FROM table1
UNION
SELECT d, e, f AS order_by, g FROM table2
ORDER BY order_by DESC
Try this :
INSERT INTO database1.table1
SELECT * FROM database2.table2
Yes you can do that, just use full database name before table, make sure your user have permissions on both databases.
SELECT ...
FROM database1.table t1
JOIN database2.table2 t2 ON t2.column = t1.col
Related
table1 (id, name)
table2 (id, name)
Query:
SELECT name
FROM table2
-- that are not in table1 already
SELECT t1.name
FROM table1 t1
LEFT JOIN table2 t2 ON t2.name = t1.name
WHERE t2.name IS NULL
Q: What is happening here?
A: Conceptually, we select all rows from table1 and for each row we attempt to find a row in table2 with the same value for the name column. If there is no such row, we just leave the table2 portion of our result empty for that row. Then we constrain our selection by picking only those rows in the result where the matching row does not exist. Finally, We ignore all fields from our result except for the name column (the one we are sure that exists, from table1).
While it may not be the most performant method possible in all cases, it should work in basically every database engine ever that attempts to implement ANSI 92 SQL
You can either do
SELECT name
FROM table2
WHERE name NOT IN
(SELECT name
FROM table1)
or
SELECT name
FROM table2
WHERE NOT EXISTS
(SELECT *
FROM table1
WHERE table1.name = table2.name)
See this question for 3 techniques to accomplish this
I don't have enough rep points to vote up froadie's answer. But I have to disagree with the comments on Kris's answer. The following answer:
SELECT name
FROM table2
WHERE name NOT IN
(SELECT name
FROM table1)
Is FAR more efficient in practice. I don't know why, but I'm running it against 800k+ records and the difference is tremendous with the advantage given to the 2nd answer posted above. Just my $0.02.
SELECT <column_list>
FROM TABLEA a
LEFTJOIN TABLEB b
ON a.Key = b.Key
WHERE b.Key IS NULL;
https://www.cloudways.com/blog/how-to-join-two-tables-mysql/
This is pure set theory which you can achieve with the minus operation.
select id, name from table1
minus
select id, name from table2
Here's what worked best for me.
SELECT *
FROM #T1
EXCEPT
SELECT a.*
FROM #T1 a
JOIN #T2 b ON a.ID = b.ID
This was more than twice as fast as any other method I tried.
Watch out for pitfalls. If the field Name in Table1 contain Nulls you are in for surprises.
Better is:
SELECT name
FROM table2
WHERE name NOT IN
(SELECT ISNULL(name ,'')
FROM table1)
You can use EXCEPT in mssql or MINUS in oracle, they are identical according to :
http://blog.sqlauthority.com/2008/08/07/sql-server-except-clause-in-sql-server-is-similar-to-minus-clause-in-oracle/
That work sharp for me
SELECT *
FROM [dbo].[table1] t1
LEFT JOIN [dbo].[table2] t2 ON t1.[t1_ID] = t2.[t2_ID]
WHERE t2.[t2_ID] IS NULL
You can use following query structure :
SELECT t1.name FROM table1 t1 JOIN table2 t2 ON t2.fk_id != t1.id;
table1 :
id
name
1
Amit
2
Sagar
table2 :
id
fk_id
email
1
1
amit#ma.com
Output:
name
Sagar
All the above queries are incredibly slow on big tables. A change of strategy is needed. Here there is the code I used for a DB of mine, you can transliterate changing the fields and table names.
This is the strategy: you create two implicit temporary tables and make a union of them.
The first temporary table comes from a selection of all the rows of the first original table the fields of which you wanna control that are NOT present in the second original table.
The second implicit temporary table contains all the rows of the two original tables that have a match on identical values of the column/field you wanna control.
The result of the union is a table that has more than one row with the same control field value in case there is a match for that value on the two original tables (one coming from the first select, the second coming from the second select) and just one row with the control column value in case of the value of the first original table not matching any value of the second original table.
You group and count. When the count is 1 there is not match and, finally, you select just the rows with the count equal to 1.
Seems not elegant, but it is orders of magnitude faster than all the above solutions.
IMPORTANT NOTE: enable the INDEX on the columns to be checked.
SELECT name, source, id
FROM
(
SELECT name, "active_ingredients" as source, active_ingredients.id as id
FROM active_ingredients
UNION ALL
SELECT active_ingredients.name as name, "UNII_database" as source, temp_active_ingredients_aliases.id as id
FROM active_ingredients
INNER JOIN temp_active_ingredients_aliases ON temp_active_ingredients_aliases.alias_name = active_ingredients.name
) tbl
GROUP BY name
HAVING count(*) = 1
ORDER BY name
See query:
SELECT * FROM Table1 WHERE
id NOT IN (SELECT
e.id
FROM
Table1 e
INNER JOIN
Table2 s ON e.id = s.id);
Conceptually would be: Fetching the matching records in subquery and then in main query fetching the records which are not in subquery.
First define alias of table like t1 and t2.
After that get record of second table.
After that match that record using where condition:
SELECT name FROM table2 as t2
WHERE NOT EXISTS (SELECT * FROM table1 as t1 WHERE t1.name = t2.name)
I'm going to repost (since I'm not cool enough yet to comment) in the correct answer....in case anyone else thought it needed better explaining.
SELECT temp_table_1.name
FROM original_table_1 temp_table_1
LEFT JOIN original_table_2 temp_table_2 ON temp_table_2.name = temp_table_1.name
WHERE temp_table_2.name IS NULL
And I've seen syntax in FROM needing commas between table names in mySQL but in sqlLite it seemed to prefer the space.
The bottom line is when you use bad variable names it leaves questions. My variables should make more sense. And someone should explain why we need a comma or no comma.
I tried all solutions above but they did not work in my case. The following query worked for me.
SELECT NAME
FROM table_1
WHERE NAME NOT IN
(SELECT a.NAME
FROM table_1 AS a
LEFT JOIN table_2 AS b
ON a.NAME = b.NAME
WHERE any further condition);
I have four db table,
tableA, tableB, tableC, tableD
I am listing records with join. These all working fine.
My query is like following.
Select DISTINCT A.*, B.loginname, B.email, C.full_name, C.address1, C.state, C.city, D.*
FROM (
tableA AS A
INNER JOIN tableB AS B ON B.user_id = A.id
INNER JOIN tableC AS C ON C.user_id = A.id
INNER JOIN tableD AS D ON D.user_id = A.id
) WHERE A.id = '269' ORDER BY A.created_date DESC
Now, issue is i have created four another table with almost same details but there are different fields and columns. So i can not use UNION.
tableE, tableF, tableG, tableH
i want to merge output to display. And output should be ORDER BY A.created_date OR ORDER BY E.created_date
Advanced thanks...
You can use UNION ALL (or UNION, if you want duplicates removed) between two tables with differently named columns by declaring name aliases.
SELECT 'first' AS source, a,b,c,d
FROM first
UNION ALL
SELECT 'second' AS source, q AS a, r AS b, s AS c, t AS d
FROM second
This will yield a result set (virtual table) with columns named source,a,b,c,d.
I'm doing some maintenance work on a database application and I've discovered that, joy of joys, even though values from one table are being used in the style of foreign keys, there's no foreign key constraints on the tables.
I'm trying to add FK constraints on these columns, but I'm finding that, because there's already a whole load of bad data in the tables from previous errors which have been naively corrected, I need to find the rows which don't match up to the other table and then delete them.
I've found some examples of this kind of query on the web, but they all seem to provide examples rather than explanations, and I don't understand why they work.
Can someone explain to me how to construct a query which returns all the rows with no matches in another table, and what it's doing, so that I can make these queries myself, rather than coming running to SO for every table in this mess that has no FK constraints?
Here's a simple query:
SELECT t1.ID
FROM Table1 t1
LEFT JOIN Table2 t2 ON t1.ID = t2.ID
WHERE t2.ID IS NULL
The key points are:
LEFT JOIN is used; this will return ALL rows from Table1, regardless of whether or not there is a matching row in Table2.
The WHERE t2.ID IS NULL clause; this will restrict the results returned to only those rows where the ID returned from Table2 is null - in other words there is NO record in Table2 for that particular ID from Table1. Table2.ID will be returned as NULL for all records from Table1 where the ID is not matched in Table2.
I would use EXISTS expression since it is more powerful, you can e.g. more precisely choose rows you would like to join. In the case of LEFT JOIN, you have to take everything that's in the joined table. Its efficiency is probably the same as in the case of LEFT JOIN with null constraint.
SELECT t1.ID
FROM Table1 t1
WHERE NOT EXISTS (SELECT t2.ID FROM Table2 t2 WHERE t1.ID = t2.ID)
SELECT id FROM table1 WHERE foreign_key_id_column NOT IN (SELECT id FROM table2)
Table 1 has a column that you want to add the foreign key constraint to, but the values in the foreign_key_id_column don't all match up with an id in table 2.
The initial select lists the ids from table1. These will be the rows we want to delete.
The NOT IN clause in the where statement limits the query to only rows where the value in the foreign_key_id_column is not in the list of table 2 ids.
The SELECT statement in parenthesis will get a list of all the ids that are in table 2.
Let we have the following 2 tables(salary and employee)
Now i want those records from employee table which are not in salary.
We can do this in 3 ways:
Using inner Join
select * from employee
where id not in(select e.id from employee e inner join salary s on e.id=s.id)
Using Left outer join
select * from employee e
left outer join salary s on e.id=s.id where s.id is null
Using Full Join
select * from employee e
full outer join salary s on e.id=s.id where e.id not in(select id from salary)
Where T2 is the table to which you're adding the constraint:
SELECT *
FROM T2
WHERE constrained_field NOT
IN (
SELECT DISTINCT t.constrained_field
FROM T2
INNER JOIN T1 t
USING ( constrained_field )
)
And delete the results.
From similar question here MySQL Inner Join Query To Get Records Not Present in Other Table I got this to work
SELECT * FROM bigtable
LEFT JOIN smalltable ON bigtable.id = smalltable.id
WHERE smalltable.id IS NULL
smalltable is where you have missing records, bigtable is where you have all the records. The query list all the records that not exist in smalltable but exists on the bigtable. You could replace id by any other matching criteria.
I Dont Knew Which one Is Optimized (compared to #AdaTheDev
) but This one seems to be quicker when I use (atleast for me)
SELECT id FROM table_1 EXCEPT SELECT DISTINCT (table1_id) table1_id FROM table_2
If You want to get any other specific attribute you can use:
SELECT COUNT(*) FROM table_1 where id in (SELECT id FROM table_1 EXCEPT SELECT DISTINCT (table1_id) table1_id FROM table_2);
You could opt for Views as shown below:
CREATE VIEW AuthorizedUserProjectView AS select t1.username as username, t1.email as useremail, p.id as projectid,
(select m.role from userproject m where m.projectid = p.id and m.userid = t1.id) as role
FROM authorizeduser as t1, project as p
and then work on the view for selecting or updating:
select * from AuthorizedUserProjectView where projectid = 49
which yields the result as shown in the picture below i.e. for non-matching column null has been filled in.
[Result of select on the view][1]
You can do something like this
SELECT IFNULL(`price`.`fPrice`,100) as fPrice,product.ProductId,ProductName
FROM `products` left join `price` ON
price.ProductId=product.ProductId AND (GeoFancingId=1 OR GeoFancingId
IS NULL) WHERE Status="Active" AND Delete="No"
SELECT * FROM First_table
MINUS
SELECT * FROM another
How to select rows with no matching entry in Both table?
select * from [dbo].[EmppDetails] e
right join [Employee].[Gender] d on e.Gid=d.Gid
where e.Gid is Null
union
select * from [dbo].[EmppDetails] e
left join [Employee].[Gender] d on e.Gid=d.Gid
where d.Gid is Null
I have table1 with 3 numbers and table2 with 1 number:
table1 table2
n n
1 1
2
3
I want to select data from table1 that is NOT present in table2 (numbes 2 and 3). I tried:
select table1.* from table1, table2 where table1.n <> table2.n
I also tried other where clauses:
where table1.n not like table2.n
where not table1.n = table2.n
But I don't get the results. I know it can be done in multiple steps, but I wonder if there is a simple query to do it. Thanks
You need to do a LEFT JOIN and look for null values for t2. Like this:
SELECT
t1.n
FROM
table1 AS t1
LEFT JOIN table2 AS t2
ON t1.n = t2.n
WHERE
t2.n IS NULL
Here is link to a great reference for different sorts of JOINS which includes Venn diagrams to help you visualize the different approaches to joining.
http://blog.codinghorror.com/a-visual-explanation-of-sql-joins/
You can do that using NOT IN or NOT EXISTS.
select * from table1
where table1.n not in (select table2.n from table2);
select * from table1
where not exists (select table2.n from table2 where table2.n = table1.n);
I tried both approaches (left join and not in) several times and it took the exact same time (my table is pretty big). Thanks guys!
I have a MYSQL query selecting from two tables. The second table may not necessarily have a matching join id to the first table. Let's use these two as examples:
++++++++++++++++++
table: t1
column: id
column: test_id
column: info
table t2
column: t2id
column: test_id
column: extra_info
++++++++++++++++++
I inserted these following rows into the table (this is just all pseudo at the moment):
insert into t1 values (1, 4, "asd")
insert into t1 values (2, 25, "dfg")
insert into t2 values (1, 25, "123")
Now my goal is to join the two tables together, but I am having issues with the join. if the second table (t2) doesn't seem to have a matching row it makes the join column in the original table NULL? Why is it doing this? Here is an example query:
SELECT * FROM `t1` LEFT JOIN `t2` ON (`t1.test_id` = `t2.test_id`) WHERE `t1.id` = 1;
Now I have absolutely no issues if they match up, but for some reason if there is no row that exists, then test_id from t1 is being set as NULL...My goal is to join these two tables if there is something to join. EDIT: If there is nothing to join then i want to just return the data from t1 by either having all the t2 columns set to NULL or just returning the t1 data. Do I need to do a subquery? and if so what would it be? Thanks :)
Use an INNER JOIN.
SELECT * FROM `t1`
INNER JOIN `t2` ON (`t1.test_id` = `t2.test_id`)
WHERE `t1.id` = 1;
Documentation on JOIN types: http://dev.mysql.com/doc/refman/5.7/en/join.html
That's the definition of LEFT JOIN. It returns rows even if only the left half of the join matches, filling the right side with NULL if needed.
If you want to get only the rows where there's a corresponding right side, use just JOIN instead of LEFT JOIN.
When joining, it is advisable not to use "SELECT *", particularly when you have similarly named columns in multiple tables, as this introduces ambiguity, and you may not get the result intended. I would suggest instead:
SELECT t1.*, t2.t2id, t2.extra_info
FROM `t1`
LEFT JOIN `t2`
ON (`t1.test_id` = `t2.test_id`)
WHERE `t1.id` = 1;
There are multiple different types of joins. I think what you want is "inner join", where only rows that have a match are returned. Check out http://www.w3schools.com/sql/sql_join.asp