Let's say I have a view named AllUsers which returns a result set of data for all users in a system. Let's say that the underlying query is somewhat complex.
Let's say that I need the ability to get a user by ID. All of the attributes should be returned for the single user as is returned for the full user set.
I'm assuming that SQL Server is smart enough to optimize performance when I apply a where clause to a view so that it optimizes the query as a single record query as opposed to selecting all records and then filtering the records based on the single ID provided in the where clause. Can you please confirm this?
You must insert a Where clause at the end of Select sentence. Example:
Select id, name, department
from AllUsers
Where id = 1
The optimization depends wether the AllUsers table have or not an index defined with the id column inner.
If the Id Column is not defined in a index SQL Server will try to scan in the whole table seeking your record.
Related
I created a table with id as primary key, firstname, lastname, email as fields. I issued the following query:
"SELECT email,COUNT(email) FROM mytable;"
The result had the first value for email i.e from the first record and the total number of values from the column 'email'. Why did it not display all the different values for email from different records?
https://dev.mysql.com/doc/refman/8.0/en/group-by-handling.html says:
Without GROUP BY, there is a single group and it is nondeterministic which name value to choose for the group.
Like with any grouping query, the COUNT(*) returns the count of rows in the group, and reduces the result to one row per group.
But since you don't have any GROUP BY clause, the whole table is treated as one big group, and the result only returns one row.
The email column returns one of the email values in the group. Technically this value is chosen arbitrarily from one of the rows in the group.
In practice, MySQL's implementation chooses the value from the first row read from the group, the order of which depends on which index was used to scan the table. Though this behavior of picking the first row is not documented and they make no guarantee to make this behavior the same from version to version.
It's better to avoid depending on queries that return an arbitrary, implementation-dependent value. It makes your code harder to maintain, because there's a risk it will change if you upgrade MySQL and they change their undocumented behavior.
To protect you from making these sorts of arbitrary queries, newer versions of MySQL enforce an SQL mode called ONLY_FULL_GROUP_BY. That option makes it an error to run a query like that one you show. For what it's worth, the query is already an error in most other brands of SQL database.
I'm having a table with an id and a name.
I'm getting a list of id's and i need their names.
In my knowledge i have two options.
Create a forloop in my code which executes:
SELECT name from table where id=x
where x is always a number.
or I'm write a single query like this:
SELECT name from table where id=1 OR id=2 OR id=3
The list of id's and names is enormous so i think you wouldn't want that.
The problem of id's is the id is not always a number but a random generated id containting numbers and characters. So talking about ranges is not a solution.
I'm asking this in a performance point of view.
What's a nice solution for this problem?
SQLite has limits on the size of a query, so if there is no known upper limit on the number of IDs, you cannot use a single query.
When you are reading multiple rows (note: IN (1, 2, 3) is easier than many ORs), you don't know to which ID a name belongs unless you also SELECT that, or sort the results by the ID.
There should be no noticeable difference in performance; SQLite is an embedded database without client/server communication overhead, and the query does not need to be parsed again if you use a prepared statement.
A "nice" solution is using the INoperator:
SELECT name from table where id in (1,2,3)
Also, the IN operator is syntactic sugar built for exactly this purpose..
SELECT name from table where id IN (1,2,3,4,5,6.....)
Hoping that you are getting the list of ID's on which you have to perform a query for names as input temp table #InputIDTable,
SELECT name from table WHERE ID IN (SELECT id from #InputIDTable)
Here's the problem:
I have an Access query that feeds a report, which sometimes doesn't return any records for certain criteria. I would like to display zeros in the report instead of an empty line (an empty recordset is currently being returned).
Is there an SQL solution that (perhaps using some kind of union statement and/or nested SQL) always returns one record (with zeros) if there are not matching records from the initial query?
One possible solution would be to create a second table with the same primary key, and add just one record. In your query, choose as join type all records in the second table, including those with no matching records in the first one. Select as output all fields in the first table.
You can materialize a one-row table with zero for all columns. This is a slight pain to achieve in Access (ACE, Jet, whatever) because it doesn't support row constructors and the FROM must resolve to a base table. In other words, you'll need a table that is guaranteed to always contain at least one row.
This isn't a problem for me because my databases always include auxilliary tables e.g. a calendar table, a sequence table of integers, etc. For exmaple, to materialize a table one-row, all-zeros table using my 3000 row Calendar table:
SELECT DISTINCT 0 AS c
FROM Calendar;
I can then UNION my query with my materialized table but include an antijoin to ensure the all-zeros row only appears in the resultset when my query is the empty set:
SELECT c
FROM T
UNION
SELECT 0
FROM Calendar
WHERE NOT EXISTS (
SELECT c
FROM T
);
Note the use of UNION allows me to remove the DISTINCT keyword and the AS clause ("column alias") from the materialized table.
I have one a SOQL query. Field list can include a subquery if the query traverses a relationship. For example:
SELECT Account.Name, (SELECT Contact.LastName FROM Account.Contacts) FROM Account
I couldn't understand the line 'traverses a relationship', can anyone explain it?
I believe they're saying that you can use the SELECT to return fields from tables that have a relationship to the table in your FROM clause, if they have a relationship and you use a subquery. In your example, you are able to use the subquery to return "LastName" from the "Contact" table even though your FROM is pulling from "Account", because there is a relationship there ("Account.Contacts").
In this instance it means you can recall a one to many relationship from the "many" side inside a query from the "one" side object. In effect it is similar to the nested query in SQL querying a subtable based on a current position of a cursor in top query. The difference is mostly in the result set begin flat in SQL versus being hierarchical in SOQL.
To make it even plainer, this query means: foreach Account load name and traverse all its contacts to retrieve contacts' last names. If there is one account with 10 contacts, you'll get one primary row and its Contacts list will contain 10 rows
not a valid sql. A subquery should have a join otherwise it will return more than one record this would violate the rule of one record per row of the main query
I'm dealing with two tables which have 2 columns, as listed under.
Table 1: table_snapshot
account_no | balance_due
Table 2: table_ paid
account_no | post_balance | delta_balance
I added a third column to table2 with the following command:
ALTER TABLE table_paid ADD delta_balance number(18);
I'm trying to use the following query, to update the new column ( delta_balance ) with the difference in balances between 1 and 2.
FYI, table_paid is a subset of table_snapshot. i,e., table 2 has only a few accounts present in table 1. I get an error saying : SQL Statement not properly ended. the query i'm using is:
UPDATE table_paid
SET table_paid.delta_balance = table_paid.post_balance - table_snapshot.balance_due
from table_paid, table_snapshot
WHERE table_paid.account_no = table_snapshot.account_no;
Appreciate if someone can correct my query.
Many thanks.
novice.
Oracle doesn't have the UPDATE ... FROM syntax that you're using from MS Sql Server (which, I believe, isn't ANSI anyway). Instead, when you need to do an update on a result set, Oracle has you create the resultset as a kind of inline view, then you update through the view, like so:
UPDATE ( SELECT tp.delta_balance
, tp.post_balance
, ts.balance_due
FROM table_paid tp
JOIN table_snapshot ts
ON tp.account_no = ts.account_no
)
SET delta_balance = post_balance - balance_due;
This is more "correct" than the answers supplied by Babar and palindrom, as their queries will update every row in table_paid, even if there are no corresponding rows in table_snapshot. If there is a 1-1 correspondance, you don't need to worry, but it's safer to do it with the inline view.
It's unclear from your example which table is the parent table, or (as I'm guessing) neither is the parent table and account_no is pointing to the primary key of another table (presumably account, or "table_account" by your naming conventions). In any case, it's clear that there is not a 1-1 correspondence in your table - 15K in one, millions in the other.
This could mean 2 things: either there are many rows in table_snapshot that have no corresponding row in table_paid, or there are many rows in table_snapshot for each row in table_paid. If the latter is true, your query is impossible - you will have multiple updates for each row in table_paid, and the result will be unpredictable; how will you know which of the "post_balance - balance_due" expressions will ultimately determine the value of a given delta_balance?
If you run my query, you will find this out quickly enough - you will get an error message that says, "ORA-01779: cannot modify a column which maps to a non key-preserved table". This error will appear based not on the data in the table (it may be okay), but based on the primary keys you have defined on the two tables. If the join condition you specify doesn't unambiguously result in a 1-1 relationship between the updated table and the rest of the join, based on the defined keys, you will get this error. It's Oracle's way of telling you, "You're about to screw up your data".
In the other answers here, you will only get an error (in that case, ORA-01427: single-row subquery returns more than one row) if you actually have data that would cause a problem; my version is more strict, so it may turn out that you will need to use the other versions.
And, as the others have said, you'll definitely want an index on account_no for the table_snapshot table. One on the table_paid wouldn't hurt either.
Try this
UPDATE table_paid
SET table_paid.delta_balance = table_paid.post_balance -
(SELECT table_snapshot.balance_due from table_snapshot WHERE table_paid.account_no =
table_snapshot.account_no);
UPDATE table_paid
SET table_paid.delta_balance = table_paid.post_balance - ( select balance_due from table_snapshot
WHERE table_paid.account_no = table_snapshot.account_no )