I'm trying to get make a query to return a list of the column names from a query result in postgresql
e.g.
SELECT column_name
FROM (
SELECT table1.*
FROM table1
LEFT JOIN table2
ON table1.id = table2.tbl1_id
)
is this possible
I DO NOT WANT THE COLUMNS FROM A SINGULAR TABLE!!! so please dont tell me to do
SELECT column_name
FROM information_schema.columns
WHERE table_name = 'table1'
I don't know why you would ever need to do this, but it is possible using some of the JSON functions introduced in 9.3.
SELECT json_object_keys(row_to_json(t)) FROM
(SELECT * FROM table1
LEFT JOIN table2 ON table1.id = table2.tbl1_id LIMIT 1) t;
This will give you the name of every column returned for a single row. Without the LIMIT you would get the columns repeated for every row returned. If you wanted to see the values returned as well you can get more complex:
WITH t as
(SELECT * FROM table1
LEFT JOIN table2 ON table1.id = table2.tbl1_id LIMIT 1)
SELECT json_data.key, json_data.value
FROM t, json_each_text(row_to_json(t)) AS json_data;
Both these queries will return all the columns even if they are named the same. If all you want is a list of unique column names, you can utilize hstore:
CREATE EXTENSION hstore; --Create the extension if you need it.
SELECT akeys(hstore(t)) as array_of_columns
FROM
(SELECT * FROM table1
LEFT JOIN table2 ON table1.id = table2.tbl1id LIMIT 1) t;
Maybe there is a less hacky way, but I think this should work:
create table query_column_sample from (your query where 1=2);
SELECT column_name
FROM information_schema.columns
WHERE table_name = 'query_column_sample'
Querying tables returns data. Querying the information schema returns metadata. Column names are metadata.
SELECT 'table1', column_name
FROM information_schema.columns
WHERE table_catalog = 'database_name'
AND table_schema = 'schema_name'
AND table_name = 'table1'
UNION ALL
SELECT 'table2', column_name
FROM information_schema.columns
WHERE table_catalog = 'database_name'
AND table_schema = 'schema_name'
AND table_name = 'table2';
"schema_name" is probably public. But be warned that you can have multiple tables with the same name, as long as they are in different schemas. (You could have "public.table1" and "private.table1".) You can also have multiple tables with the same name in different databases. You need to specify all three values--database name, schema name, table name--to be sure of getting the right data.
If I had to do this in production I'd probably output more data. I'd also probably include the column "ordinal_position", and sort on it. Sorting by "ordinal_position" will order column names as they appear in the current table definition.
SELECT table_catalog
, table_schema
, table_name
, column_name
, ordinal_position
FROM information_schema.columns
WHERE table_catalog = 'sandbox'
AND table_schema = 'public'
AND table_name IN ('table1', 'table2')
ORDER BY table_catalog
, table_schema
, table_name
, ordinal_position;
The only reasonable approach I can come up with is to do something like this. Schema location plays into this, but I'm just posting the basic idea. SQL doesn't have reflection at run time unless you go and hit meta data tables, which is what you seem to be trying to achieve here.
SELECT table1.*
INTO x
FROM table1
LEFT JOIN table2
ON table1.id = table2.tbl1_id LIMIT 0;
select *
from information_schema.columns
where table_Name = 'x';
Also interesting enough while traveling the Internets over lunch I ran into this. Works with PostgreSQL 8.2.0 or higher.
https://github.com/theory/colnames/tree/master/test/sql
Related
For example, I have table 1 and table 2, is there a way I can check the which columns from table 1 exist in table 2? Thanks!
While the best solutions will be RDBMS specific (which you have not indicated), a general way that's supported by most databases is to use the information_schema views, for example the following will list the column names in table1 that are also in table2:
select column_name
from information_schema.columns t1
where t1.table_schema='schemaName' and t1.table_name='table1'
and exists (
select * from information_schema.COLUMNS t2
where t2.column_name = t1.column_name
and t2.table_schema = t1.table_schema
and t2.column_name ='table2'
);
Most of the tables in the DB have a certain column 'col_abc'.
How can I find the tables that do not contain this column 'col_abc'.
The query
select table_name
from user_tab_columns
where column_name not in ('COL_ABC')
does not return expected result.
Thanks,
Amit
Here is one way:
select table_name from user_tables
minus
select table_name from user_tab_columns
where column_name = 'ABC';
The Below Query will give the expected results. Please modify <ur_Schema_name> as the required schema name
SELECT * FROM ALL_TABLES at
WHERE at.OWNER = <ur_Schema_name>
AND NOT EXISTS (SELECT 1 FROM ALL_TAB_COLUMNS atc
WHERE atc.TABLE_NAME = at.TABLE_NAME
AND atc.OWNER =at.OWNER
AND atc.COLUMN_NAME='COL_ABC'
AND atc.OWNER=<ur_Schema_name>);
If you only need to look in the current user's schema, this is probably the simplest way to write the query:
select table_name
from user_tables
where table_name not in (
select table_name
from user_tab_columns
where column_name = 'COL_ABC'
)
;
This uses in a crucial way the fact that the column table_name in user_tab_columns is constrained not null.
If you need to look across all schemas, you need to use the dba_ or all_ dictionary views, and look for (owner, table_name) pairs instead of just table_name. Left as further exercise for you.
You could also use this solution using LEFT JOIN. It is a kind of NOT EXISTS clause.
select t.*
from user_tables t
left join (
select table_name
from user_tab_columns
where column_name = 'COL_ABC'
) tc
on (t.table_name = tc.table_name)
where tc.table_name is null
;
I am working with two very wide tables (Table A, Table B) in Snowflake. Both tables should have exact same columns, though not necessarily in the same order. However Table B is missing a few columns. Is there a way to quickly find all the columns that are in Table A but not in Table B? I am not concerned with the Data, just the existence of columns.
Essentially, I am looking for a SQL way to find all columns in Table that are missing in Table B.
I currently use a ETL tool for this. But it is rather cumbersome. It would be nice if this can be done using SQL.
Try the following
SELECT column_name
FROM information_schema.columns
WHERE table_name = 'tableA'
AND column_name NOT IN
(
SELECT column_name
FROM information_schema.columns
WHERE table_name = 'tableB'
);
You can do this with aggregation:
SELECT column_name
FROM information_schema.columns
WHERE table_name IN ('tableA', 'tableB')
GROUP BY column_name
HAVING MIN(table_name) = MAX(table_name) AND MIN(table_name) = 'TableA';
This is a handy structure that can more easily generalize to multiple tables. It does assume that the two tables are on the same server.
Below query also gives missing columns in Table B.
SELECT column_name FROM information_schema.columns WHERE table_name = 'TableA'
MINUS
SELECT column_name FROM information_schema.columns WHERE table_name = 'TableB'
I have a database and have 2 schema called DWFR and DWFR3 , what i need to do is compare 2 schmea and take the matching table name and also the table present in DWFR schema which is not available on DWFR3 and also with the below condition
SELECT schema_name, table_name
FROM tables
WHERE schema_name = 'DWFR'
AND table_name NOT LIKE 'RA%'
AND table_name NOT LIKE 'ZZ%'
AND table_name NOT LIKE 'DELETE%'
AND table_name NOT LIKE '%TEST%'
AND table_name NOT LIKE 'XX%'
AND table_name NOT LIKE 'ROB%'
AND table_name IN (
SELECT table_name
FROM tables
WHERE schema_name = 'DWFR3'
AND table_name NOT LIKE 'RA%'
AND table_name NOT LIKE 'ZZ%'
AND table_name NOT LIKE 'DELETE%'
AND table_name NOT LIKE '%TEST%'
AND table_name NOT LIKE 'XX%'
AND table_name NOT LIKE 'ROB%'
)
ORDER BY schema_name, table_name;
This will get the matched table names but i also want table name available in DWFR which is not their on DWFR3.
You can use a LEFT OUTER JOIN for selecting all data rows from a set and matchings from a second set.
I used a SQL CTE Common Table Expression in the script, you can think of it as a sub-select with additional uses
Here is how you can use for your case
with t2 as (
select table_name
from tables
where schema_name = 'DWFR3'
)
select t1.table_name, t2.table_name
from tables t1
left outer join t2
on t1.table_name = t2.table_name
where t1.schema_name = 'DWFR'
And you also need the append the additional requirements or WHERE clauses into above script
For the reason that I used CTE can be explained as follows.
In fact, the actual solution should be as simple as following.
Unfortunately, HANA seems to not supporting additional filtering criteria with ON clause on JOINs
select t1.table_name, t2.table_name
from tables t1
left outer join tables t2
on t1.table_name = t2.table_name and t2.schema_name = 'DWFR3
where t1.schema_name = 'DWFR'
Because of this reason, I had to filter the LEFT part of the JOIN earlier than applying the ON, join condition
To find matched table names in both schemas:
select
'DWFR' as schema_name,
table_name
from
(
select table_name from tables where schema_name = 'DWFR'
intersect
select table_name from tables where schema_name = 'DWFR3'
) x
where
table_name not like 'RA%' and
table_name not like 'ZZ%' and
table_name not like 'DELETE%' and
table_name not like '%TEST%' and
table_name not like 'XX%' and
table_name not like 'ROB%';
To find table names in DWFR but not in DWFR3:
select
'DWFR' as schema_name,
table_name
from
(
select table_name from tables where schema_name = 'DWFR'
except
select table_name from tables where schema_name = 'DWFR3'
) x
where
table_name not like 'RA%' and
table_name not like 'ZZ%' and
table_name not like 'DELETE%' and
table_name not like '%TEST%' and
table_name not like 'XX%' and
table_name not like 'ROB%';
In SQL Server how do you query a database to bring back all the tables that have a field of a specific name?
The following query will bring back a unique list of tables where Column_Name is equal to the column you are looking for:
SELECT Table_Name
FROM INFORMATION_SCHEMA.COLUMNS
WHERE Column_Name = 'Desired_Column_Name'
GROUP BY Table_Name
SELECT Table_Name
FROM Information_Schema.Columns
WHERE Column_Name = 'YourFieldName'
I'm old-school:
SELECT DISTINCT object_name(id)
FROM syscolumns
WHERE name = 'FIELDNAME'