Can anyone explain to me why they use AS id and AS r2 and USING id on the codes :
SELECT id, username FROM friendship JOIN (SELECT CEIL(RAND() * (SELECT MAX(id) FROM friendship)) AS id) AS r2 USING (id);
I just want to know the purpose of using them, why using them?
The As is just to create an Alias for the subquery, that way you can use that same alias 'id' with the using command.
Basically the query wants the friendship table to join the subquery on the id column, since the subquery only returns one result it can join with the id column in the friendship table.
OscarMk already explained the AS, but the USING is the equivalent of ON table1.id = table2.id. By using USING (a bit redundant here...), your RDBMS basically merge the columns into a single one, which means you don't have to tell your RDBMS if you want the column from table1 or table2. Simple example :
SELECT table1.id
FROM table1
INNER JOIN table2
ON table1.id = table2.id;
SELECT id
FROM table1
INNER JOIN table2
USING (id);
Those queries are equivalent. If you had forgotten to use table1.id in the first query, your RDBMS would have raised an error.
You can usually do a NATURAL JOIN instead of a INNER JOIN, and not use a USING or ON clause at all (that's what NATURAL JOIN are for)
Related
i would like to know if there is any shortcut to specify the column where IN have to check for matches.
Example:
Instead of this:
select *
from table1
where id in(
select column
from table2
)
something like this:
select *
from table1
where id in table2.column
I know the existence of TABLE for IN, ANY, SOME to specify a table, but it works only if the table specified is composed by just 1 column
EDIT: using join is not an option, because the real use i was looking for is on a NOT IN operator, and also JOIN create duplicates sometimes like in a one to many relation
There is no shortcut like that in SQL. Let me explain why.
In a query, all table references need to be made in the FROM clause. Hence, you cannot simply refer to table2.col unless table2 has been defined in the FROM clause. table2 is actually an alias, which defaults to the table name.
From a performance perspective, I would recommend exists:
select t1.*
from table1 t1
where exists (select column
from table2 t2
where t2.column = t1.id
)
In particular, this can take advantage of an index on table2(column) and has the same semantics as in.
Using a JOIN is a bit shorter. At least it does not require a subquery or another SELECT ... FROM.
SELECT table1.*
FROM table1
JOIN table2 ON table1.id = table2.column
Although this simple example is an inner join, not a semi-join. An inner join is different because it produces one row per matched row in table2. A semi-join only produces one row for each row in table1, even if it matches multiple rows in table2.
If you want to simulate a semi-join, use DISTINCT to reduce the result to one row per row of table1:
SELECT DISTINCT table1.*
FROM table1
JOIN table2 ON table1.id = table2.column
If you want to check for something like NOT EXISTS, use an exclusion join:
SELECT table1.*
FROM table1
LEFT OUTER JOIN table2 ON table1.id = table2.column
WHERE table2.column IS NULL
No need to use DISTINCT on the outer join example. There will be no row duplication from the join, because it can only "match no rows" once.
I am not sure if this is possible. But is it possible to do a join on 2 tables, but return the data for only one of the tables. I want to join the two tables based on a condition, but I only want the data for one of the tables. Is this possible with SQL, if so how? After reading the docs, it seems that when you do a join you get the data for both tables. Thanks for any help!
You get data from both tables because join is based on "Cartesian Product" + "Selection". But after the join, you can do a "Projection" with desired columns.
SQL has an easy syntax for this:
Select t1.* --taking data just from one table
from one_table t1
inner join other_table t2
on t1.pk = t2.fk
You can chose the table through the alias: t1.* or t2.*. The symbol * means "all fields".
Also you can include where clause, order by or other join types like outer join or cross join.
A typical SQL query has multiple clauses.
The SELECT clause mentions the columns you want in your result set.
The FROM clause, which includes JOIN operations, mentions the tables from which you want to retrieve those columns.
The WHERE clause filters the result set.
The ORDER BY clause specifies the order in which the rows in your result set are presented.
There are a few other clauses like GROUP BY and LIMIT. You can read about those.
To do what you ask, select the columns you want, then mention the tables you want. Something like this.
SELECT t1.id, t1.name, t1.address
FROM t1
JOIN t2 ON t2.t1_id = t1.id
This gives you data from t1 from rows that match t2.
Pro tip: Avoid the use of SELECT *. Instead, mention the columns you want.
This would typically be done using exists (or in) if you prefer:
select t1.*
from table1 t1
where exists (select 1 from table2 t2 on t2.x = t1.y);
Although you can use join, it runs the risk of multiplying the number of rows in the result set -- if there are duplicate matches in table2. There is no danger of such duplicates using exists (or in). I also find the logic to be more natural.
If you join on 2 tables.
You can use SELECT to select the data you want
If you want to get a table of data, you can do this,just select one table date
SELECT b.title
FROM blog b
JOIN type t ON b.type_id=t.id;
If you want to get the data from two tables, you can do this,select two table date.
SELECT b.title,t.type_name
FROM blog b
JOIN type t ON b.type_id=t.id;
I have two joined tables:
SELECT table1.*, table2.* FROM table1 t1 INNER JOIN table2 t2 ON t1.id = t2.t1_id
The question: In query results, id will always be taken from secondary table defined in SELECT statement?
For example:
if I use select t1.*, t2.* - in results id will be t2.id
if I use select t2.*, t1.* - id will be t1.id.
Is this good practice to use 'merged' result, or should I avoid ambiguity, and always define columns strictly?
No, the sql query will return all columns with the same name from all tables, not just the last one, unless you use a natural join (table1 inner join table2 using(column)).
If you use some kind of a component that stores the results in associative arrays, then these components usually use only the field names as key, therefore they return only the last column's value from those that have the same name.
However, it is a good practive to use an alias if you want to return more than 1 column that has the same name in the database.
My suggestion is to use tablename with alias and get to use like this. It would be best practice to run query.Mention your column names even though it has many columns. You can order your display.
SELECT t1.columnName1, t2.columnName2 FROM tablename1 t1 INNER JOIN tablename2 t2 ON t1.id = t2.id
I've never done an inner join SQL statement before, so I don't even know if this is the right thing to use, but here's my situation.
Table 1 Columns: id, course_id, unit, lesson
Table 2 Columns: id, course_id
Ultimately, I want to count the number of id's in each unit in Table 1 that are also in Table 2.
So, even though it doesn't work, maybe something like....
$sql = "SELECT table1.unit, COUNT( id ) as count, table2.id, FROM table1, table2, WHERE course_id=$im_course_id GROUP BY unit";
I'm sure the syntax of what I'm wanting to do is a complete fail. Any ideas on fixing it?
SELECT unit, COUNT( t1.id ) as count
FROM table1 as t1 inner JOIN table2 as t2
ON t1.id = t2.id
GROUP BY unit
hope this helps.
If I understand what you want (maybe you could post an example input and output?):
SELECT unit, COUNT( id ) as count
FROM table1 as t1 JOIN table2 as t2
ON t1.id = t2.id
GROUP BY unit
Okay, so there are a few things going on here. First off, commas as joins are deprecated so they may not even be supported (depending on what you are using). You should probably switch to explicitly writing inner join
Now, whenever you have any sort of join, you also need on. You need to tell sql how it should match these two tables up. The on should come right after the join, like this:
Select *
From table1 inner join table2
on table1.id = table2.id
and table1.name = table2.name
You can join on as many things as you need by using and. This means that if the primary key of one table is several columns, you can easily create a one-to-one match between tables.
Lastly, you may be having issues because of other general syntax errors in your query. A comma is used to separate different pieces of information. So in your query,
SELECT table1.unit, COUNT( id ) as count, table2.id, FROM ...
The comma at the end of the select shouldn't be there. Instead this should read
SELECT table1.unit, COUNT( id ) as count, table2.id FROM ...
This is subtle, but the sql query cannot run with the extra comma.
Another issue is with the COUNT( id ) that you have. Sql doesn't know which id to count since table1 and table2 both have ids. So, you should use either count(table1.id) or count(table2.id)
I want to do a sql query and have some problems:
I want to salect from table_1 the ID's Where parent_id is the value I have:
SELECT ID FROM table_1 WHERE parent_ID = 'x'
I want to use the ID'S I got in 1. and
SELECT FROM table_2 WHERE ID = 'The ID's from Query 1.
Simple and Execute
Select t1.`id` FROM table t1 INNER JOIN table t2 ON t1.`id`=t2.`id`
As Bainternet mentioned you can do this with a subquery
SELECT * FROM table_2 WHERE ID IN (SELECT ID FROM table_1 WHERE parent_ID = 'x')
Though your idea to use an inner join is also good (especially since MySQL can be slow when processing subqueries).
SELECT t2.* FROM table_2 as t2 INNER JOIN table_1 AS t1 ON t2.ID = t1.ID WHERE t1.parent_ID = 'x'
If that's not clear, try looking at the MySQL JOIN Syntax or the Subqueries, as Bainternet mentioned. If these examples and the MySQL docs aren't clear enough for you, consider posting more details on exactly what you're trying to do (e.g. include table structures in your question). Also while you might want this information for some WordPress related work you are doing, there's nothing in the question itself that actually ties it to WordPress. So if you have more questions that about MySQL queries in general, then you might want to consider posting them to the StackOverflow, tagged as mysql-query.