I have two tables (Table 1 and Table 2) that I join to then do a coalesce. So...
SELECT t1.*,
coalesce(t1.A, t2.A_T) as A
FROM table1 as t1
INNER JOIN table2 as t2
ON t1.key = t2.key
Doing what I said above, I will get a repeated A column given I want to replace missing values in A from Table 1 with Table 2's A column. I want to do a union but a repeated A column is an issue (as seen in Table 3). You cannot do a union with different numbers of columns.
How can I fix this issue?
What I want:
Note: the actual tables I am working with have over 40 columns each. But this example is to get to the point of my issue.
Snowflake supports exclude so you could do
select coalesce(t1.A, t2.A_T) as A, t1.* exclude(t1.A)
from ...
Note that your problem stems from the fact that you are using * to select columns instead of being explicit about which columns to select.
Related
I have 8 tables all with equal number of columns and with a common column. I want to fetch data from all tables in a single query.
My table structure is TABLE1, TABLE2, TABLE3, ..... TABLE 8.
that have columns COLUMNA, COLUMNB... COLUMNE and a COMMON_COLUMN
I need to get data with a where clause where COMMON_COLUMN='X'
I will need all columns from all tables.
I used a query that goes like this..
SELECT TABLE1.*, TABLE2.*, TABLE3.*
FROM TABLE1 T1
LEFT JOIN TABLE2 T2 ON T1.COMMON_COLUMN = T2.COMMON_COLUMN,
LEFT JOIN TABLE3 T3 ON T1.COMMON_COLUMN = T3.COMMON_COLUMN
WHERE T1.COMMON_COLUMN='X' AND T2.COMMON_COLUMN='X' AND T3.COMMON_COLUMN='X'
The above query is not giving any results even if one of the tables do not have any rows. I do not want to use inner join because although the tables have a common column they do not depend on each other and I need data from all tables with a certain common column.
Also, the tables have unequal number of rows.
What am I doing wrong?
correct me if i am wrong - as you do not attach any sample data and desired result
but i assume that you simply need union all tables. You write in the title that tables are independent
SELECT T1.*
FROM TABLE1 T1
WHERE T1.COMMON_COLUMN='X'
UNION ALL
SELECT T2.*
FROM TABLE2 T2
WHERE T2.COMMON_COLUMN='X'
UNION ALL
SELECT T3.*
FROM TABLE3 T3
WHERE T3.COMMON_COLUMN='X'
...
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);
Back in the old days, I used to write select statements like this:
SELECT
table1.columnA, table2.columnA
FROM
table1, table2
WHERE
table1.columnA = 'Some value'
However I was told that having comma separated table names in the "FROM" clause is not ANSI92 compatible. There should always be a JOIN statement.
This leads to my problem.... I want to do a comparison of data between two tables but there is no common field in both tables with which to create a join. If I use the 'legacy' method of comma separated table names in the FROM clause (see code example), then it works perfectly fine. I feel uncomfortable using this method if it is considered wrong or bad practice.
Anyone know what to do in this situation?
Extra Info:
Table1 contains a list of locations in 'geography' data type
Table2 contains a different list of 'geography' locations
I am writing select statement to compare the distances between the locations. As far I know you cant do a JOIN on a geography column??
You can (should) use CROSS JOIN. Following query will be equivalent to yours:
SELECT
table1.columnA
, table2.columnA
FROM table1
CROSS JOIN table2
WHERE table1.columnA = 'Some value'
or you can even use INNER JOIN with some always true conditon:
FROM table1
INNER JOIN table2 ON 1=1
Cross join will help to join multiple tables with no common fields.But be careful while joining as this join will give cartesian resultset of two tables.
QUERY:
SELECT
table1.columnA
, table2,columnA
FROM table1
CROSS JOIN table2
Alternative way to join on some condition that is always true like
SELECT
table1.columnA
, table2,columnA
FROM table1
INNER JOIN table2 ON 1=1
But this type of query should be avoided for performance as well as coding standards.
A suggestion - when using cross join please take care of the duplicate scenarios. For example in your case:
Table 1 may have >1 columns as part of primary keys(say table1_id,
id2, id3, table2_id)
Table 2 may have >1 columns as part of primary keys(say table2_id,
id3, id4)
since there are common keys between these two tables (i.e. foreign keys in one/other) - we will end up with duplicate results. hence using the following form is good:
WITH data_mined_table (col1, col2, col3, etc....) AS
SELECT DISTINCT col1, col2, col3, blabla
FROM table_1 (NOLOCK), table_2(NOLOCK))
SELECT * from data_mined WHERE data_mined_table.col1 = :my_param_value
I have two datasets on two seperate servers. They both pull one column of information each.
I would like to build a report showing the values of the rows that only appear in one of the datasets.
From what I have read, it seems I would like to do this on the SQL side, not the reporting side; I am not sure how to do that.
If someone could shed some light on how that is possible, I would really appreciate it.
You can use the NOT EXISTS clause to get the differences between the two tables.
SELECT
Column
FROM
DatabaseName.SchemaName.Table1
WHERE
NOT EXISTS
(
SELECT
Column
FROM
LinkedServerName.DatabaseName.SchemaName.Table2
WHERE
Table1.Column = Table2.Column --looks at equalities, and doesn't
--include them because of the
--NOT EXISTS clause
)
This will show the rows in Table1 that don't appear in Table2. You can reverse the table names to find the rows in Table2 that don't appear in Table1.
Edit: Made an edit to show what the case would be in the event of linked servers. Also, if you wanted to see all of the rows that are not shared in both tables at the same time, you can try something as in the below.
SELECT
Column, 'Table1' TableName
FROM
DatabaseName.SchemaName.Table1
WHERE
NOT EXISTS
(
SELECT
Column
FROM
LinkedServerName.DatabaseName.SchemaName.Table2
WHERE
Table1.Column = Table2.Column --looks at equalities, and doesn't
--include them because of the
--NOT EXISTS clause
)
UNION
SELECT
Column, 'Table2' TableName
FROM
LinkedServerName.DatabaseName.SchemaName.Table2
WHERE
NOT EXISTS
(
SELECT
Column
FROM
DatabaseName.SchemaName.Table1
WHERE
Table1.Column = Table2.Column
)
You can also use a left join:
select a.* from tableA a
left join tableB b
on a.PrimaryKey = b.ForeignKey
where b.ForeignKey is null
This query will return all records from tableA that do not have corresponding records in tableB.
If you want rows that appear in exactly one data set and you have a matching key on each table, then you can use a full outer join:
select *
from table1 t1 full outer join
table2 t2
on t1.key = t2.key
where t1.key is null and t2.key is not null or
t1.key is not null and t2.key is null
The where condition chooses the rows where exactly one match.
The problem with this query, though, is that you get lots of columns with nulls. One way to fix this is by going through the columns one by one in the SELECT clause.
select coalesce(t1.key, t2.key) as key, . . .
Another way to solve this problem is to use a union with a window function. This version brings together all the rows and counts the number of times that key appears:
select t.*
from (select t.*, count(*) over (partition by key) as keycnt
from ((select 'Table1' as which, t.*
from table1 t
) union all
(select 'Table2' as which, t.*
from table2 t
)
) t
) t
where keycnt = 1
This has the additional column specifying which table the value comes from. It also has an extra column, keycnt, with the value 1. If you have a composite key, you would just replace with the list of columns specifying a match between the two tables.
I have two databases, for argument sake lets call them db1 and db2. they are both structured exactly the same and both have a table called table1 which both have fields id and value1.
My question is how do I do a query that selects the field value1 for both tables linked by the same id???
You can prefix the table names with the database name to identify the two similarly named tables. You can then use that fully qualified table name to refer to the similarly named fields.
So, without aliases:
select db1.table1.id, db1.table1.value1, db2.table1.value1
from db1.table1 inner join db2.table1 on db1.table1.id = db2.table1.id
and with aliases
select t1.id, t1.value1, t2.value1
from db1.table1 as t1 inner join db2.table1 as t2 on t1.id = t2.id
You may also want to alias the selected columns so your select line becomes:
select t1.id as id, t1.value1 as value_from_db1, t2.value1 as value_from_db2
This is T-Sql, but I can't imagine mysql would be that much different (will delete answer if that's not the case)
SELECT
a.Value1 AS [aValue]
,b.Value1 AS [bValue]
FROM
db1.dbo.Table1 a
INNER JOIN db2.dbo.Table1 b
ON a.Id = b.Id
Try something such as this.
$dbhost="server_name";
$dbuser1="user1";
$dbpass1="password1";
$dbname1="database_I";
$dbname2="database_II";
$db1=mssql_connect($dbhost,$dbuser1,$dbpass1);
mssql_select_db($dbname1,$db1);
$query="SELECT ... FROM database_I.table1, database_II.table2 WHERE ....";
etc. Sorry if this does not help.
There is an easy way in sql. Extend your syntax for FROM clause, so instead of using select ... from tablename, use
select ... from database.namespace.tablename
The default namespace is dbo.
You could use a union select:
Simple example:
select "one" union select "two";
This will return 2 rows, the first row contains one and the 2nd row contains two. It is as if you are concatenating 2 sql quires, the only constant is that they both must return the same number of columns.
Multiple databases:
select * from client_db.users where id=1 union select * from master_db.users where id=1;
In this case both users databases must have the same number of columns. You said they have the same structure, so you shouldn't have a problem.