To demonstrate what I need, this query doesn't work:
SELECT
dbo.expensive_function AS test,
IIF(test=1, 3, 4) AS test2
But this does:
SELECT
*,
IIF(test=1, 3, 4) AS test2
FROM (
SELECT
dbo.expensive_function AS test
)
Is there a cleaner way of doing this? I'm translating legacy code from Microsoft Access to SQL Server and I'm finding that I need multiple nested tables to achieve the desired results. It's also undesirable to run the expensive function twice in the query.
There does not seem to be any correlation between your query and the expensive function.
Execute the function before the query, store the value in a variable and use the variable in the query instead of the expensive function.
Related
I'm using a PostgreSQL with a Go driver. Sometimes I need to query not existing fields, just to check - maybe something exists in a DB. Before querying I can't tell whether that field exists. Example:
where size=10 or length=10
By default I get an error column "length" does not exist, however, the size column could exist and I could get some results.
Is it possible to handle such cases to return what is possible?
EDIT:
Yes, I could get all the existing columns first. But the initial queries can be rather complex and not created by me directly, I can only modify them.
That means the query can be simple like the previous example and can be much more complex like this:
WHERE size=10 OR (length=10 AND n='example') OR (c BETWEEN 1 and 5 AND p='Mars')
If missing columns are length and c - does that mean I have to parse the SQL, split it by OR (or other operators), check every part of the query, then remove any part with missing columns - and in the end to generate a new SQL query?
Any easier way?
I would try to check within information schema first
"select column_name from INFORMATION_SCHEMA.COLUMNS where table_name ='table_name';"
And then based on result do query
Why don't you get a list of columns that are in the table first? Like this
select column_name
from information_schema.columns
where table_name = 'table_name' and (column_name = 'size' or column_name = 'length');
The result will be the columns that exist.
There is no way to do what you want, except for constructing an SQL string from the list of available columns, which can be got by querying information_schema.columns.
SQL statements are parsed before they are executed, and there is no conditional compilation or no short-circuiting, so you get an error if a non-existing column is referenced.
I am trying to make workload test in Jmeter.
This is Postgresql and I would like to combine a random variable with '%'.
Could you help me the way to make sql query?
I have already tried with '?%', but there is an error "The column index is out of range: 1, number of columns: 0.".
The easiest way is switching to Select Statement and amend your query to look like:
SELECT * FROM scloud_usr.tcus_usr_rgn where UID like '${__RandomString(3,ABD,UID)}%'
If you want to continue with the Prepared Select Statement you need to add this % wildcard to the parameter value itself, not in the query, i.e.
More information:
Using “like” wildcard in prepared statement
Performance Testing BLOB from a MySQL Database with JMeter
Prepared statements with % wildcards
I would like to be able to execute an explain statement on a query that has bind parameters. For example:
EXPLAIN SELECT * FROM metasyntax WHERE id = $1;
When I attempt to execute this, I get the following error:
ERROR: bind message supplies 0 parameters, but prepared statement "" requires 1
I understand it's telling me that it wants me to supply a value for the query. However, I may not necessarily know the answer to that. In other SQL dialects such as Oracle, it will generate an explain plan without needing me to supply a value for the parameter.
Is it possible to get an explain plan without binding to an actual value? Thanks!
Assuming the parameter is an integer:
PREPARE x(integer) AS SELECT * FROM metasyntax WHERE id = $1;
Then run the following six times, where “42” is a representative value:
EXPLAIN (ANALYZE, BUFFERS) EXECUTE x(42);
Normally PostgreSQL will switch to a generic plan for the sixth run, and you will see a plan that contains the placeholder $1.
No.
The optimiser is allowed to change the query plan based on the parameter. Imagine if the parameter was null - it's obvious that no rows will be returned, so the DB may return an empty rowset instantly.
Just use a representative value.
For example, I'd like to run:
REGEXP_REPLACE("What's My Name?", "[^a-z0-9_\-]", "-");
and simply see what it returns, instead of doing a search against a DB Table. I tried to run it in the CLI and got
ERROR: syntax error at or near "REGEXP_REPLACE"
LINE 1: REGEXP_REPLACE("What's My Name?", "[^a-z0-9_\-]", "-")
(I'm trying to be generic- I'd like to be able to use this for other PSQL Aggregate Functions as well.)
Remember, this is SQL, so every output you get is a relation. Hence to calculate the result of a function, you need to run SELECT to retrieve the function's value.
Unfortunately, in many DBs, SELECT requires a table. In Oracle land, there's dual to work around this problem:
SELECT REGEXP_REPLACE('What''s My Name?', '[^a-z0-9_\-]', '-') FROM dual;
PostgreSQL, however, allows you to execute a SELECT query without having to specify a table:
SELECT REGEXP_REPLACE('What''s My Name?', '[^a-z0-9_\-]', '-');
Note that the string quote in SQL is ', not ". PostgreSQL uses double quotes to quote identifiers.
Side note: Not every function is an aggregate. An aggregate is a function that combines multiple values into a single output value. REGEXP_REPLACE() is just a normal string function.
Is there a way to run a single PSQL Aggregate Function without hitting a database table?
Yes, in PostgreSQL you can write a SELECT statement without a FROM part at all.
I'm trying to construct a T-SQL statement with a WHERE clause determined by an input parameter. Something like:
SELECT * FROM table
WHERE id IN
CASE WHEN #param THEN
(1,2,4,5,8)
ELSE
(9,7,3)
END
I've tried all combination of moving the IN, CASE etc around that I can think of. Is this (or something like it) possible?
try this:
SELECT * FROM table
WHERE (#param='??' AND id IN (1,2,4,5,8))
OR (#param!='??' AND id in (9,7,3))
this will have a problem using an index.
The key with a dynamic search conditions is to make sure an index is used, instead of how can I easily reuse code, eliminate duplications in a query, or try to do everything with the same query. Here is a very comprehensive article on how to handle this topic:
Dynamic Search Conditions in T-SQL by Erland Sommarskog
It covers all the issues and methods of trying to write queries with multiple optional search conditions. This main thing you need to be concerned with is not the duplication of code, but the use of an index. If your query fails to use an index, it will preform poorly. There are several techniques that can be used, which may or may not allow an index to be used.
here is the table of contents:
Introduction
The Case Study: Searching Orders
The Northgale Database
Dynamic SQL
Introduction
Using sp_executesql
Using the CLR
Using EXEC()
When Caching Is Not Really What You Want
Static SQL
Introduction
x = #x OR #x IS NULL
Using IF statements
Umachandar's Bag of Tricks
Using Temp Tables
x = #x AND #x IS NOT NULL
Handling Complex Conditions
Hybrid Solutions – Using both Static and Dynamic SQL
Using Views
Using Inline Table Functions
Conclusion
Feedback and Acknowledgements
Revision History
if you are on the proper version of SQL Server 2008, there is an additional technique that can be used, see: Dynamic Search Conditions in T-SQL Version for SQL 2008 (SP1 CU5 and later)
If you are on that proper release of SQL Server 2008, you can just add OPTION (RECOMPILE) to the query and the local variable's value at run time is used for the optimizations.
Consider this, OPTION (RECOMPILE) will take this code (where no index can be used with this mess of ORs):
WHERE
(#search1 IS NULL or Column1=#Search1)
AND (#search2 IS NULL or Column2=#Search2)
AND (#search3 IS NULL or Column3=#Search3)
and optimize it at run time to be (provided that only #Search2 was passed in with a value):
WHERE
Column2=#Search2
and an index can be used (if you have one defined on Column2)
if #param = 'whatever'
select * from tbl where id in (1,2,4,5,8)
else
select * from tbl where id in (9,7,3)