I've got an Sqlite database where one of the columns is defined as "TEXT NOT NULL". Some of the values are strings and some can be cast to a DOUBLE and some can be case to INTEGER. Once I've narrowed it down to DOUBLE values, I want to do a query that gets a range of data. Suppose my column is named "Value". Can I do this?
SELECT * FROM Tbl WHERE ... AND Value >= 23 AND Value < 42
Is that going to do some kind of ASCII comparison or a numeric comparison? INTEGER or REAL? Does the BETWEEN operator work the same way?
And what happens if I do this?
SELECT MAX(Value) FROM Tbl WHERE ...
Will it do string or integer or floating-point comparisons?
It is all explained in the Datatypes In SQLite Version 3 article. For example, the answer to the first portion of questions is
An INTEGER or REAL value is less than any TEXT or BLOB value. When an INTEGER or REAL is compared to another INTEGER or REAL, a numerical comparison is performed.
This is why SELECT 9 < '1' and SELECT 9 < '11' both give 1 (true).
The expression "a BETWEEN b AND c" is treated as two separate binary comparisons "a >= b AND a <= c"
The most important point to know is that column type is merely an annotation; SQLite is dynamically typed so each value can have any type.
you cant convert text to integer or double so you wont be able to do what you want.
If the column were varchar you could have a chance by doing:
select *
from Tbl
WHERE ISNUMERIC(Value ) = 1 --condition to avoid a conversion from string to int for example
and cast(value as integer) > 1 --rest of your conditions
Related
Is there a sane way of storing int, float and boolean values in the same column in Postgres?
If have something like that:
rid
time
value
2d9c5bdc-dfc5-4ce5-888f-59d06b5065d0
2021-01-01 00:00:10.000000 +00:00
true
039264ad-af42-43a0-806b-294c878827fe
2020-01-03 10:00:00.000000 +00:00
2
b3b1f808-d3c3-4b6a-8fe6-c9f5af61d517
2021-01-01 00:00:10.000000 +00:00
43.2
Currently I'm using jsonb to store it, the problem however now is, that I can't filter in the table with for instance the greater operator.
The query
SELECT *
FROM points
WHERE value > 0;
gives back the error:
ERROR: operator does not exist: jsonb > integer: No operator matches the given name and argument types. You might need to add explicit type casts.
For me it's okay to handle boolean as 1 or 0 in case of true or false. Is there any possibility to achieve that with jsonb or is there maybe another super type which lets me use a column that is able to use all three types?
Performance is not so much of a concern here, as I'm going to have very few records inside of that table, max 5k I guess.
If you were just storing integers and floats, normally you'd use a float or numeric column.
But there's that pesky true.
You could cast the JSON...
select *
from test
where value::float > 1;
...but there's that pesky true.
You have to convert the boolean to a number to make it work.
select *
from test
where
(case when value = 'true' then 1.0 when value = 'false' then 0.0 else value::float end) >= 1;
Or ignore it.
This having to work around the type system suggests that value is actually two or even three different fields crammed into one. Consider separating them into multiple columns.
You should skip the rows where value is not number and cast the value to numeric, e.g.:
with points(id, value) as (
values
(1, 'true'::jsonb),
(2, '2'),
(3, '43.2')
)
select *
from points
where jsonb_typeof(value) = 'number'
and value::text::numeric > 0;
id | value
----+-------
2 | 2
3 | 43.2
(2 rows)
I actually found out, regardless of the jsonb fields value, that you can compare it to other jsonb in postgres. That means, I can for instance do the following:
SELECT *
FROM points
WHERE val > '5'
This correctly gives me back only the third row. It just ignores the bool value. To filter for a certain bool I can achieve that with the following query:
SELECT *
FROM points
WHERE val = 'true'
This is good enough for me. I even could hold timestamps in the json column and compare them using this methodology.
Another way of solving the problem after all your comments seem to be to make the column a numeric. This would work as well, but requires more client side conversion, as I would have to have a second type column, remembering what the actual type is. This type should than be used on the client side to convert the value back into its og value. For integers its trivial, for booleans like #schwern suggested, one can use 1 and 0, for dates, one could use the unix timestamp representation.
When I now want to search for a certain value, the type has to be contained in the where clause as well.
Using SQL in PostgreSQL I need to select all the rows from my table called "crop" when the first digit of the integer numbers in column "field_id" is 7.
select *
from crop
where (left (field_id,1) = 7)
First, you know that the column is a number, so I would be inclined to explicitly convert it, no matter what you do:
where left(crop::text, 1) = '7'
where crop::text like '7%'
The conversion to text is simply to be explicit about what is happening and it makes it easier for Postgres to parse the query.
More importantly, if the value has a fixed number of digits, then I would suggest using a numeric range; something like this:
where crop >= 700000 and crop < 800000
This makes it easier for Postgres to use an index on the column.
Try with cast, like this:
select *
from crop
where cast(substring(cast(field_id as varchar(5)),1,1) as int) = 7
where 5 in varchar(5) you should put number how long is your integer.
I have a table with a field named MINIMUM_AGE. The values stored in this field are of type nvarchar:
17 years
54 years
N/A
65 years
I would like to apply a WHERE clause on the column to check for a certain age range. To do that I need to parse out the age from the field values.
So, I think I need to select the first two characters, then cast them into an integer. Also, some fields may not contain numbers for the first two characters. Some may simply be N/A. So, I will need to check for that before casting.
Can someone explain how to accomplish this?
Here is the SQL Fiddle that demonstrates the below query:
SELECT CASE
WHEN MINIMUM_AGE <> 'N/A'
THEN CAST(LEFT(MINIMUM_AGE, 2) AS int)
ELSE 0
END
FROM MyTable
Note: the CASE expression can only return one data type. So, in the example above if the MINIMUM_AGE is N/A then it returns 0.
If you would rather have it return null, then use the following:
SELECT CASE
WHEN MINIMUM_AGE <> 'N/A'
THEN CAST(LEFT(MINIMUM_AGE, 2) AS int)
END
FROM MyTable
I have a nvarchar(200) column in a table that contains a mix of integers (as strings) and non-integer strings and symbols also. E.g. Some sample data :-
Excuse me for my typing in my initial post I mentioned varchar(200) but in fact it is 'nvarchar(200)'
02
0
.../
125
00125
/2009
1000
0002589
000.00125
Marathi numbers like & letters
how can I order this Column?
You can use CAST to convert a varchar to an INT given that varchar is holding a proper number.
SELECT CAST(varCharCol as Int)
E.g.
col1 as Varchar(10)
col1 = '100' converting to INT will be successufl
but if col1 = '100 xyz' will be unsucessful in the process.
Looking at your string you may have to use number of LTRIM, REPLACE to get hold of the digits or using a regex to get comma separated numbers. That too it's not very clear how your original string looks like.
References.
Many DBMS have CAST() functions where you can convert one datatype to another.
For MySQL have a look at this site
You can Use CAST and Convert to convert string type value to int type. but be sure the value should numeric.
select convert(int,'123')
select CAST('123' as int)
You can use this query
SELECT CASE
WHEN ISNUMERIC(colName)=1 THEN ROUND(colName, 0)
ELSE 0 END AS [colName]
FROM tblName
This was my Interview Question
there are two columns called Length and Breadth in Area table
Length Breadth Length*Breadth
20 NULL ?
30 ?
21.2 1 ?
I tried running the same question on MYSQL while inserting,To insert an empty value I tried the below query . Am I missing anything while inserting empty values in MYSQL.
insert into test.new_table values (30,);
Answers: With Null,Result is Null.
With float and int multiplication result is float
As per your question the expected results would be as below.
SELECT LENGTH,BREADTH,LENGTH*BREADTH AS CALC_AREA FROM AREA;
LENGTH BREADTH CALC_AREA
20
30 0 0
21.2 1 21.2
For any(first) record in SQL SERVER if you do computation with NULL the answer would be NULL.
For any(second) record in SQL SERVER, if you do product computation between a non-empty value and an empty value the result would be zero as empty value is treated as zero.
For any(third) record in SQL SERVER, if you do computation between two non-empty data type values the answer would be a NON-EMPTY value.
Check SQL Fiddle for reference - http://sqlfiddle.com/#!3/f250a/1
That blank Breath (second row) cannot happen unless Breath is VARCHAR. Assuming that, the answers will be:
NULL (since NULL times anything is NULL)
Throws error (since an empty string is not a number. In Sql Server, the error is "Error converting data type varchar to numeric.")
21.20 (since in Sql Server, for example, conversion to a numeric type is automatic, so SELECT 21.2 * '1' returns 21.20).
Assuming that Length and Breadth are numerical types of some kind the second record does not contain possible values — Breadth must be either 0 or NULL.
In any event, any mathematical operation in SQL involving a NULL value will return the value NULL, indicating that the expression cannot be evaluated. The answer are NULL, impossible, and 21.2.
The product of any value and NULL is NULL. This is called "NULL propagation" if you want to Google it. To score points in an interview, you might want to mention that NULL isn't a value; it's a special marker.
The fact that the column Breadth has one entry "NULL" and one entry that's blank (on the second row) is misleading. A numeric column that doesn't have a value in a particular row means that row is NULL. So the second column should also show "NULL".
The answer to the third row, 21.2 * 1, depends on the data type of the column "Length*Breadth". If it's a data type like float, double, or numberic(16,2), the answer is 21.2. If it's an integer column (integer, long, etc.), the answer is 21.
A more snarky answer might be "There's no answer. The string "Length*Breadth" isn't a legal SQL column name."
In standard SQL they would all generate errors because you are comparing values (or nulls) of different types:
CAST ( 20 AS FLOAT ) * CAST ( NULL AS INTEGER ) -- mismatched types error
CAST ( '' AS INTEGER ) -- type conversion error
CAST ( AS INTEGER ) -- type conversion error
CAST ( 21.2 AS FLOAT ) * CAST ( 2 AS INTEGER ) -- mismatched types error
On the other hand, most SQL product would implicitly cast values when comparing values (or nulls) of different types according to type precedence e.g. comparing float value to an integer value would in effect cast the integer to float and result in a float. At the product level, the most interesting question is what happens when you compare a null of type integer with a value (or even a null) of type float...
...but, frankly, not terribly interesting. In an interview you are presented with a framework (in the form of questions asked of you) on which to present your knowledge, skills and experience. The 'answer' here is to discuss nulls (e.g. point out that nulls are tricky to define and behave in unintuitive ways, which leads to frequent bugs and a desire to avoid nulls entirely, etc) and whether implicit casting is a good thing.