SELECT * FROM table t
SELECT t.* FROM table t
I tried it and it yielded the same results, but I want to make sure because I'm refactoring a piece of code that uses the second version, and I was surprised as it is both longer to write, and less simple.
Are there any hidden stuff here?
MySQL version: 5.5.29-0ubuntu0.12.04.2 (Ubuntu)
Both statements are the same in your case.
They would be not if you join multiple tables in one query.
select *
selects all columns.
select t.*
select all columns of table t (or the table assigned the alias t)
SELECT * FROM table t and SELECT t.* FROM table t
Return the whole table
SELECT t.* FROM table as t inner join table2 as t2
will only return the fields in the "table" table while
SELECT * FROM table as t inner join table2 as t2
will return the fields of table and table2
Both the statements will give same results until it's combined with another table with some table operator as Join, Apply where you will need to uniquely identify columns( more specifically ambiguous columns ) from this table.
As a best practice you should use column names instead of using select * as it makes code more readable and front end code doesn't break in case table structure gets changed at any point of time.
The statements are identical. All you have is an alias for table "table" called "t".
SELECT * will return all columns from all tables in the query. SELECT t.* will return all columns from the table named, or aliased as, t. The same in your example because there's only one table involved.
Related
I am new to mySQL.
I am following Mosh's tutorial to familiarize myself to SQL.
Here's my question for the following code.
SELECT *
FROM order_items
WHERE order_id = 6 AND unit_price*quantity > 30
When I looked up about SELECT *, it says: * means to return all all columns of the queried tables. Then I think SELECT * means that it grabs all tables from all schema.
My question is: Isn't it a bit inefficient and confusing to return all column provided my understanding is right? If the database become bigger and bigger, it will consume unnecessary effort to look up the keyword, so I think SELECT should specify what table it is referring to. Thanks for reading! 🥰
SELECT * does not fetch all tables from all schema. It only fetches the columns from the table you reference in your FROM clause. It only fetches the rows that match your WHERE clause.
The mistake is understandable given this statement in the MySQL documentation:
A select list consisting only of a single unqualified * can be used as shorthand to select all columns from all tables:
SELECT * FROM t1 INNER JOIN t2 ...
What they mean by "all tables" is only all tables referenced in this query. And only those in FROM or JOIN clauses. Not all tables everywhere.
Let's say I have a table named "mytable01". When I want to have two instances of the table in the query I do this:
SELECT t1.column, t2.column
FROM mytable01 t1, mytable01 t2;
Now, there are times that I want two instances from the same nested SELECT. Is there a way to do it without having to write it two times?
No! You want to do a self-join (joining the table with itself) you must to declare it each time. By the way try to avoid write queries in implicit format and use explicit join pattern like it:
select t1.column, t2.column
from table t1
join table t2 on t2.id = t1.parentid
btw again, you example missed the columns for the join and that ill force a cross join.
Hi i have this query but its giving me an error of Operand should contain 1 column(s) not sure why?
Select *,
(Select *
FROM InstrumentModel
WHERE InstrumentModel.InstrumentModelID=Instrument.InstrumentModelID)
FROM Instrument
according to your query you wanted to get data from instrument and instrumentModel table and in your case its expecting "from table name " after your select * .when the subselect query runs to get its result its not finding table instrument.InstrumentModelId inorder to fetch result from both the table by matching you can use join .or you can also select perticuler fields by tableName.fieldName and in where condition use your condition.
like :
select Instrument.x,InstrumentModel.y
from instrument,instrumentModel
where instrument.x=instrumentModel.y
You can use a join to select from 2 connected tables
select *
from Instrument i
join InstrumentModel m on m.InstrumentModelID = i.InstrumentModelID
When you use subqueries in the column list, they need to return exactly one value. You can read more in the documentation
as a user commented in the documentation, using subqueries like this can ruin your performance:
when the same subquery is used several times, mysql does not use this fact to optimize the query, so be careful not to run into performance problems.
example:
SELECT
col0,
(SELECT col1 FROM table1 WHERE table1.id = table0.id),
(SELECT col2 FROM table1 WHERE table1.id = table0.id)
FROM
table0
WHERE ...
the join of table0 with table1 is executed once for EACH subquery, leading to very bad performance for this kind of query.
Therefore you should rather join the tables, as described by the other answer.
select *
from AllUK
where exists (select * from AllCompanies where replace(AllUK.mobile,' ','')=replace(AllCompanies.mobile,' ',''))
I need to include the columns from the AllCompanies table in to my first select. How can I do that?
select *
from AllUK a
join AllCompanies b
on a.mobile = b.mobile
exists is a boolean operation, so the clause you have above will always return all the results if there any records that can be joined accross the 2 tables. It's hard to tell what you're really trying to achieve.
Also, putting string operations on columns within exists and joins is not best practice because the compiler has to do the operation on every row & column at run time. Might be better to create a temp table to hold the replaced values and then join on that.
I was wondering if there is a way to do something like selecting all without ... some columns here
something like SELECT */column1,column2 , is there a way to do this ?
I just need to output something like
column1 , column2 ( from another table ) , here all other columns without column1 ( or something to make the select skip the first few columns)
EDIT:
The thing is that i need this to be dynamic , so i cant just select what i don't know. I never know how many columns there will be , i just know the 1st and the 2nd column
EDIT: here is a picture http://oi44.tinypic.com/xgdyiq.jpg
I don't need the second id column , just the last column like i have pointed.
Start building custom views, which are geared aorund saving developers time and encapsulating them from the database schema.
Oh, so select all but certain fields. You have two options.
One is a little slow.. Copy the table, drop the fields you don't want, then SELECT *
The other is to build the field list from a subquery to information_schema or something, then remove occurrences of 'field_i_dont_want' in that list.
SELECT ( SELECT THE TABLES YOU WANT AND CONCAT INTO ONE STRING ) FROM TABLE
If you need to combine records from multiple tables, you need to find a way to relate them together. Primary Keys, Foreign Keys, or anything common among this.
I will try to explain this with a sql similar to your problem.
SELECT table1.id, table2.name, table1.column3, table1.column4
FROM table1
INNER JOIN table2 On table2.commmonfield = table1.commonfield
If you have 'n' columns in your table as in Col1,Col2,Col3....Coln you can select whatever columns you want to select from the table.
SELECT Col1,Col2 FROM YOURTABLE;
You either select all columns (*) or especify the columns you want one by one. There is no way to select 'all but some'.
The SQL language lets you either select a wildcard set of columns or enumerated single columns from a singular table. However you can join a secondary table and get a wildcard there.
SELECT
a.col1,
b.*
FROM
table_a as a
JOIN table_b as b ON (a.col5 = b.col_1)