I'm unable to find such example in the web. I'm not an oracle user (I've been using mysql). As I read in the web, you shall use "FROM DUAL" in Oracle, when the data is not fetched from a real table (a function, for example). But how what can I do when I want to attach current date, e.g. SYSDATE when retrieving some real table data?
Let's say I've got an A table with col1 and col2 columns. Is any of the following correct? If not, please write a correct one:
SELECT col1, col2, SYSDATE
FROM A;
SELECT col1, col2, TO_CHAR(SYSDATE, 'MM-DD-YYYY HH24:MI:SS')
FROM A;
SELECT col1, col2, TRUNC(SYSDATE)
FROM A;
SELECT col1, col2, TRUNC(SYSDATE)
FROM A, DUAL;
Unfortunately, I don't have an oracle installation to test this... that's why I write it here.
This is a bit long for a comment.
They all look correct to me. The fourth uses comma in the from clause. I encourage you to never use commas in the from clause. Instead, always use explicit join syntax, replacing the comma with a cross join. However, the use of dual is pretty useless in that particular query.
You should learn about SQL Fiddle (www.sqlfiddle.com). This is a place where you can try out your code on several different databases.
Related
is it possible to use an array of elements as a select statement?
I know it is possiible to get rows based on static elements like this:
SELECT 405, CAST('4D6178' AS VARCHAR(32)), CAST('2017-01-01 00:00:00' AS TIMESTAMP) FROM rdb$databas
That will give you a table select with one row.
Now I would like to get this as table with n rows, but I don't know how to achieve this. Due to the fact that firebird doesn't allow multiple select statements I cannot only append n times a selec.
Info : Firebird 2.1
Use UNION ALL clause.
https://en.wikipedia.org/wiki/Set_operations_(SQL)#UNION_operator
Select x,y,z From RDB$DATABASE
UNION ALL
Select a,b,c From RDB$DATABASE
UNION ALL
Select k,l,m From RDB$DATABASE
Notice however that this should only be used for small data. Firebird query length is limited to 64KB and even if that would not be so - abusing this method to inject lots of data would not be good.
If you really need to enter lot of similar (same structure) data rows - use Global Temporary Tables
The following discussion hopefully would give you more insights:
https://stackoverflow.com/a/43997801/976391
I tried selecting records from a table using the query:
select *
from TableA
where trim(DateField)='31-Dec-2015'
which returned 0 rows.
However, leaving put the trim function gave me records matching my where condition. Can't point put the error.
I think you are confusing the trim and trunc functions. You also have data conversion issues. Dates should be compared to dates. By using trim you are forcing oracle to turn the date field into a string.
select * from TableA where trunc(DateField) = to_date('31-Dec-2015','dd-Mon-yyyy');
Better would be:
select *
from TableA
where DateField between to_date('31-Dec-2015','dd-Mon-yyyy')
and to_date('31-Dec-2015 23:59:59', 'dd-Mon-yyyy hh24:mi:ss');
I have 2 tables in 2 different schemas scha schb (e.g) and in scha I have a several tables that are all made of varchar as I had to format some data [it was part of the task]
now I have the same tables but with different types in schb.
The problem is this, Wherever I have a type which involves numbers (money, numerical, date), it's giving me an error to CAST.
Is there a way where I can CAST, without the need of copying one coloumn after another (copying it all in one go)
for example
INSERT INTO schb.customer
SELECT "col1", "col2" "col3 **(needs casting)**...."
FROM scha.customer
Thanks
A SELECT clause is not a list of columns, it is a list of expressions (which usually involve columns). A type cast is an expression so you can put them right into your SELECT. PostgreSQL supports two casting syntaxes:
CAST ( expression AS type )
expression::type
The first is standard SQL, the :: form is PostgreSQL-specific. If your schb.customer.col3 is (for example) numeric(5,2), then you'd say:
INSERT INTO schb.customer (col1, col2, col3)
SELECT col1, col2, cast(col3 as numeric(5,2))
FROM scha.customer
-- or
INSERT INTO schb.customer (col1, col2, col3)
SELECT col1, col2, col3::numeric(5,2)
FROM scha.customer
Note that I've included the column list in the INSERT as well. You don't have to do that but it is a good idea as you don't have to worry about the column order and it makes it easy to skip columns (or let columns assume their default values with explicitly telling them to).
This question already has answers here:
SQL column names and comparing them to row records in another table in PostgreSQL
(3 answers)
Closed 9 years ago.
I am trying to have a SQL statement where the column names in the SELECT are a subquery. The basic format is:
SELECT (<subquery for columns>) FROM Table;
My subquery returns 4 rows of field names, so I need to make them a single row. I used:
SELECT array_to_string(array_agg(column_names::text),',') FROM Fieldnames;
And then I get a returned format of col1, col2, col3, col4 for my 4 returned rows as a string. If I paste in the raw test for my query, it works fine as:
SELECT (col1, col2, col3, col4) FROM Table;
The issue arises when I put the two together. I get an odd response from psql. I get a:
?column?
col1, col2, col3, col4
with no rows returned for:
SELECT(SELECT array_to_string(array_agg(column_names::text),',') FROM Fieldnames) FROM Table;
Conceptually, I think there are two ways I can address this. I need to get my subquery SELECT back in a format that I can put as the column-name argument to the first SELECT statement, but because I return multiple rows (of a single value of a varchar for the column name that I want), I thought I could just paste them together but I cannot. I am using psql so I do not have the "#" list trick.
Any advice would be appreciated.
Solution:
Here is why the question is not a duplicate, and how I solved it. In trying to simplify the question to be manageable, it lost its muster. I ended up writing a function because I couldn't use # to pass a list to SELECT in PostgreSQL. When you want to select only a subset of rows, you cannot pass a nested (SELECT) even with an AS, although this works in Oracle. As a result, I wrote a function that effective created a string, and then passed it as the SELECT. There seems to be something fundamentally different on how the SQL parser in PostgreSQL handles the arguments for SELECT from Oracle, but everyone DB is different.
If you enclose several column names in parentheses like you do:
SELECT (col1, col2, col3, col4) FROM tbl;
.. you effectively create an ad-hoc row type from the enclosed columns, which has no name, because you did not provide an alias. Postgres will choose a fallback like ?column?. In later Postgres versions the default name is row since, internally, the above is short syntax for:
SELECT ROW(col1, col2, col3, col4) FROM tbl;
Provide your own name (alias):
SELECT (col1, col2, col3, col4) AS my_row_type FROM tbl;
But you probably just want individual columns. Drop the parentheses:
SELECT col1, col2, col3, col4 FROM tbl;
I'm using SQLite database and I'm wondering whether I'm allowed to write queries as follows:
SELECT SUM(column1 * column2)
FROM my_table;
I googled but references say that SUM function is has the following format:
SUM([DISTINCT|ALL] column)
And my question is: does column mean actually column or does it allow expressions (like above) too?
You can always use a table expression:
SELECT SUM(Calc)
FROM (
SELECT Column1 * Column2 AS 'Calc'
FROM My_Table) t
I don't have SQLite but checking the docs indicates this should work fine.
Yes, you can use an expression like the one you mentioned, if the datatype of both columns allow it.