group by SQL statement winds up all rows based on a column. Then you use aggregate functions to output something based on the wound up rows.
Sometimes you don't want to wind up all the rows. You may want them to display as normal. But calculate a value on a load of rows (say, a 'window' of rows...).
This is where
Window functions come into play. They allow you to specify a load of (or window of) rows on which to use a aggregate function.
Let's say we have this table.
name | department | salary -------+------------+-------- chris | IT | 30000 jason | IT | 35000 kate | IT | 22000 david | sales | 40000 matt | sales | 45000 james | sales | 50000
We could use this
group by SQL statement to output salary averages of the departments.
select department, avg(salary) from employees group by department; department | avg ------------+-------------------- IT | 29000.000000000000 sales | 45000.000000000000
But we may not want to wind all the columns up, just diplay an average column after each row. In that case:
select name, department, salary, avg(salary) over (partition by department) from employees; name | department | salary | avg -------+------------+--------+-------------------- chris | IT | 30000 | 29000.000000000000 jason | IT | 35000 | 29000.000000000000 kate | IT | 22000 | 29000.000000000000 david | sales | 40000 | 45000.000000000000 matt | sales | 45000 | 45000.000000000000 james | sales | 50000 | 45000.000000000000
We use the 'over' keyword to specify the column we want to partition our window by.
If you have a constraint you wish to alter, you need to drop it and re-add it.
First let's look at the constraint with
\d+ youtable ... Foreign-key constraints: "constraint_name" FOREIGN KEY (some_id) REFERENCES yourtable(some_id)
We can we see the, in this case, foreign key constraint we want to alter. We'll use text above later.
Now let's drop the constraint.
alter table yourtable drop constraint constraint_name;
Now let's copy the text we saw above, and paste it after the text
alter table yourtable add constraint.
alter table yourtable add constraint "constraint_name" FOREIGN KEY (some_id) REFERENCES yourtable(some_id) on delete cascade;
In our case, we altered the constraint by adding
on delete cascade.
SELECT tables.table_name FROM information_schema.tables WHERE tables.table_schema = 'public' AND tables.table_name != 'schema_version' AND tables.table_type = 'BASE TABLE';
The above lists all the tables, excluding schema_version since we don't care about migration info and including only those in the public schema, the ones we created, excluding views by specifying the base table table type.
SELECT columns.table_name, columns.column_name, columns.data_type, columns.column_default, columns.is_nullable FROM information_schema.columns;
The above lists all the column names in your database, their type, the column default (you can work out if it's got a sequence etc) and if they're nullable
SELECT kcu.constraint_name, kcu.table_name, kcu.column_name FROM information_schema.key_column_usage kcu LEFT JOIN information_schema.table_constraints tc ON tc.constraint_name = kcu.constraint_name WHERE tc.constraint_type = 'FOREIGN KEY';
The above lists all the foreign key constraints in your database, with the table name and column name which they belong to (not what they reference)
SELECT constraint_name, table_name, column_name FROM information_schema.constraint_column_usage;
The above lists all the constraints (foreign keys for example), along with the table name and column name which they reference.
First add the squeeze backports to /etc/apt/sources.list:
deb http://backports.debian.org/debian-backports squeeze-backports main
apt-get update apt-get -t squeeze-backports install postgresql-9.1
Then login as the postgres user and start up psql:
su - postgres psql
Now create a new user with a new password and create a database as that user:
create user myuser password 'mypassword' create database mydatabase owner myuser; \q
Now you've exited psql, log out of the postgres unix user, and attach the the postgres program remotely:
<ctrl-d> psql -h localhost -U myuser -d mydatabase
Now it'll ask you to enter in the password 'myuser'. You can now start issuing sql commands. Or ? to see help for psql commands.
You can't easily do this in sqlite. Say you have a table with a schema:
CREATE TABLE ANOTHERTHING (ID NUMBER(10) NOT NULL, TEXT VARCHAR, PRIMARY KEY (ID));
And you want to rename the column 'text' to 'sometext', you need to make a new table with that schema, rename the current table, copy all the data over into the new table and then drop the renamed table:
alter table anotherthing rename to anotherthing_1; create table anotherthing (id number(10) not null, sometext varchar, primary key (id)); insert into anotherthing(id, sometext) select id, text from anotherthing_1; drop table anotherthing_1;