I mean the following:
I have 2 parent tables :
table1
id PRIMARY KEY
name TEXT
table2
id PRIMARY KEY
...
and a child table, used fo n-n Relations :
table_child
id PRIMARY KEY
id_1 INT
id_2 INT
where id_1 and id_2 in table_child refer to the column id in table1 and table2.
Now : i often perform request, with a join between table_1 and table_child ON table1.id = table_child.id1, only because i need the value of the column table1.name.
I'm wondering if there is a way to avoid these joins, and declare somehow a "pseudo" column name in table_child, which would not be a real column, but a link to the corresponding column in table_1, so that :
* I can acces the value through table_child.name
* But it is always synchronized with the value table1.name
I hope my explanation was understandable...
Further to my comment above, the answer you're really looking for is something like:
CREATE VIEW
table1_child_view AS
SELECT
table1.name,
table1_child.*
FROM
table1_child
INNER JOIN
table1 ON
table1.id = table1_child.id_1
Then you can run your queries on the new view, such as:
SELECT name FROM table1_child_view WHERE ...
Related
I have got two tables:
create table student
(
studentid bigint primary key not null,
name varchar(200) not null
);
create table courseregistration
(
studentid bigint not null,
coursenamename varchar(200) not null,
isfinished boolean default false
);
--insert some data
insert into student values(1,'Dave');
insert into courseregistration values(1,'SQL',true);
Student is fetched with id, so it should be always returned in the result. Entry in the courseregistration is optional and should be returned if there are matching rows and those matching rows should be filtered on isfinished=false. This means I want to get the course regsitrations that are not finished yet. Tried to outer join student with courseregistration and filter courseregistration on isfinished=false. Note that, I still want to retrieve the student.
Trying this returns no rows:
select * from student
left outer join courseregistration using(studentid)
where studentid = 1
and courseregistration.isfinished = false
What I'd want in the example above, is a result set with 1 row student, but course rows null (because the only example has the isfinished=true). One more constraint though. If there is no corresponding row in courseregistration, there should still be a result for the student entry.
This is an adjusted example. I can tweak my code to solve the problem, but I really wonder, what is the "correct/smart way" of solving this in postgresql?
PS I have used the (+) in Oracle previously to solve similar issues.
Isn't this what you are looking for :
select * from student s
left outer join courseregistration cr
on s.studentid = cr.studentid
and cr.isfinished = false
where s.studentid = 1
db<>fiddle here
I have two tables(Table A and Table B) in a Postgres DB.
Both have "id" column in common. Table A has one column called "id" and Table B has three columns: "id, date, value($)".
For each "id" of Table A there exists multiple rows in Table B in the following format - (id, date, value).
For instance, for Table A with "id" as 1 if there exists following rows in Table B:
(1, 2018-06-21, null)
(1, 2018-06-20, null)
(1, 2018-06-19, 202)
(1, 2018-06-18, 200)
I would like to extract the most recent dated non-null value. For example for id - 1, the result should be 202. Please share your thoughts or let me know in case more info is required.
Here is the solution I went ahead with:
with mapping as ( select distinct table1.id, table2.value, table2.date, row_number() over (partition by table1.id order by table2.date desc nulls last) as row_number
from table1
left join table2 on table2.id=table1.id and table2.value is not null
)
select * from mapping where row_number = 1
Let me know if there is scope for improvement.
You may very well want an inner join, not an outer join. If you have an id in table1 that does not exist in table2 or that has only null values you will get NULL for both date and value. This is due to the how outer join works. What it says is if nothing in the right side table matches the ON condition then return NULL for each column in that table. So
with mapping as
(select distinct table1.id
, table2.value
, table2.date
, row_number() over (partition by table1.id order by table2.date desc nulls last) as row_number
from table1
join table2 on table2.id=table1.id and table2.value is not null
)
select *
from mapping
where row_number = 1;
See example of each here. Your query worked because all your test data satisfied the 1st condition of the ON condition. You really need test data that fails to see what your query does.
Caution: DATE and VALUE are very poor choice for a column names. Both are SQL standard reserved words, although not Postgres specifically. Further DATE is a Postgres data type. Having columns with names the same as datatype leads to confusion.
I need to write a sql code that probably is very simple but I am very new to it.
I need to find all the records from one table that have matching id (but no more than one) from the other table. eg. one table contains records of the employees and the second one with employees' telephone numbers. i need to find all employees with only one telephone no
Sample data would be nice. In absence of:
SELECT
employees.employee_id
FROM
employees
LEFT JOIN
(SELECT distinct on(employee_id) employee_id FROM emp_phone) AS phone
ON
employees.employee_id = phone.employee_id
WHERE
phone.employee_id IS NOT NULL;
You need a join of the 2 tables, group by employee and the condition in the having clause:
SELECT e.employee_id, e.name
FROM employees e INNER JOIN numbers n
ON e.employee_id = n.employee_id
GROUP BY e.employee_id, e.name
HAVING COUNT(*) = 1;
If there can be more than a few numbers per employee in the table with the employees' telephone numbers (calling it tel), then it's cheaper to avoid GROUP BY and HAVING which has to process all rows. Find employees with "unique" numbers using a self-anti-join with NOT EXISTS.
While you don't need more than the employee_id and their unique phone number, you don't even have to involve the employee table at all:
SELECT *
FROM tel t
WHERE NOT EXISTS (
SELECT FROM tel
WHERE employee_id = t.employee_id
AND tel_number <> t.tel_number -- or use PK column
);
If you need additional columns from the employee table:
SELECT * -- or any columns you need
FROM (
SELECT employee_id AS id, tel_number -- or any columns you need
FROM tel t
WHERE NOT EXISTS (
SELECT FROM tel
WHERE employee_id = t.employee_id
AND tel_number <> t.tel_number -- or use PK column
)
) t
JOIN employee e USING (id);
The column alias in the subquery (employee_id AS id) is just for convenience. Then the outer join condition can be USING (id), and the ID column is only included once in the result, even with SELECT * ...
Simpler with a smart naming convention that uses employee_id for the employee ID everywhere. But it's a widespread anti-pattern to use employee.id instead.
Related:
JOIN table if condition is satisfied, else perform no join
I have a composite type that looks like
CREATE TYPE member AS (
id BIGINT,
type CHAR(1)
);
I have a table that relies on this member type with an array.
CREATE TABLE relation (
id BIGINT PRIMARY KEY,
members member[]
);
I have three other tables each with a different schema (but having common id field)
CREATE TABLE table_x (
id BIGINT PRIMARY KEY,
some_text TEXT
);
CREATE TABLE table_y (
id BIGINT PRIMARY KEY,
some_int INT
);
CREATE TABLE table_z (
id BIGINT PRIMARY KEY,
some_date TIMESTAMP
);
type field in member type is just one character to find out table that specific member belongs to. A row in relation table can have a mix of different types.
I have a scenario which requires returning relation ids with at least one member fulfilling a certain condition based on it's type (let's say for x => some_text is not empty or y => some_int is greater than 10 or z => some_date is a week is from now).
I can implement this scenario on the application side by making multiple requests to the database:
unnest relation table
collect member data per relation
make new requests to find out relations
I am wondering if there is a way to map column values to table names and join them.
Assumption
I´m assuming that relation.members array does not have more than one member element of the same type. Correct?
Query to try
with unnested_members as (
-- Unnest members array
select id, unnest(members) members
from relation
)
, members_joined as (
-- left join on a per type basis with table_x, table_y and table_z.
select r.id, (r.members).id idext, (r.members).type,
x.some_text, y.some_int, z.some_date -- more types, more columns here
from unnested_members r
left join table_x x on (x.id = (r.members).id and (r.members).type = 'x')
left join table_y y on (y.id = (r.members).id and (r.members).type = 'y')
left join table_z z on (z.id = (r.members).id and (r.members).type = 'z')
-- More types, more tables to left join
)
select id,
max(some_text) some_text, -- use max() to get not null value for this id
max(some_int) some_int, -- use max() to get not null value for this id
max(some_date) some_date -- use max() to get not null value for this id
-- more types, more max() columns here
from members_joined
group by id -- get one row per relation.id with data from joined table_* columns
If you need to include more tables then you have to include these tables in the left join part, include the column in the select list and in the max() section as well.
#JNevill had a good point about this database design. Although this approach may not seem optimal, it keeps the table definitions clearly separate without any relations in between them. Also the size of relation table is fairly small compared to other three tables.
I solved the problem by simply fetching rows per type and merging them:
SELECT relation.* FROM relation, UNNEST(relation.members) member INNER JOIN table_x ON member.id = table_x.id WHERE member.type = 'x' AND table_x.some_text = 'some text value'
UNION
SELECT relation.* FROM relation, UNNEST(relation.members) member INNER JOIN table_y ON member.id = table_y.id WHERE member.type = 'y' AND table_y.some_int = 123
UNION
SELECT relation.* FROM relation, UNNEST(relation.members) member INNER JOIN table_z ON member.id = table_z.id WHERE member.type = 'z' AND table_z.some_date > '2017-01-11 00:00:00';
I have two table with one of them is vertical i.e store only key value pair with ref id from table 1. i want to join both table and dispaly key value pair as a column in select. and also perform sorting on few keys.
T1 having (id,empid,dpt)
T2 having (empid,key,value)
select
T1.*,
t21.value,
t22.value,
t23.value,
t24.value
from Table1 t1
join Table2 t21 on t1.empid = t21.empid
join Table2 t22 on t1.empid = t22.empid
join Table2 t23 on t1.empid = t23.empid
where
t21.key = 'FNAME'
and t22.key = 'LNAME'
and t23.key='AGE'
The query you demonstrate is very inefficient (another join for each additional column) and also has a potential problem: if there isn't a row in T2 for every key in the WHERE clause, the whole row is excluded.
The second problem can be avoided with LEFT [OUTER] JOIN instead of [INNER] JOIN. But don't bother, the solution to the first problem is a completely different query. "Pivot" T2 using crosstab() from the additional module tablefunc:
SELECT * FROM crosstab(
'SELECT empid, key, value FROM t2 ORDER BY 1'
, $$VALUES ('FNAME'), ('LNAME'), ('AGE')$$ -- more?
) AS ct (empid int -- use *actual* data types
, fname text
, lname text
, age text);
-- more?
Then just join to T1:
select *
from t1
JOIN (<insert query from above>) AS t2 USING (empid);
This time you may want to use [INNER] JOIN.
The USING clause conveniently removes the second instance of the empid column.
Detailed instructions:
PostgreSQL Crosstab Query