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
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 am using mySQL 5.6. I have two tables: t1 and t2. Both have many columns. And many columns in t1 and t2 share the same name: for example, there is a "var1" column in t1 and in t2.
I want to join the tables, selecting (a) all columns from t1 and (b) only the columns in t2 that have names that don't appear in t1. For example, I would not select "var1" from t2.
Here is a valid mySQL command that does not work because some columns share the same name:
SELECT * FROM t1 LEFT JOIN t2 ON (t1.ID=t2.ID);
MySQL sensibly returns this error message:
ERROR 1060 (42S21): Duplicate column name 'var1'
So I want to run a command like
SELECT t1.*, DISTINCTCOLUMNS(t2.*) FROM t1 LEFT JOIN t2 ON (t1.ID=t2.ID);
Except, of course, that there is no DISTINCTCOLUMNS operation in SQL. But is there a similar (real) command that will achieve the same effect?
I see that common advice is to avoid SELECT * syntax, partly for efficiency purposes. I appreciate that, but I do not want to write out the names of all of the columns that I need.
The real issue here is you have the same column name for both tables. So you need to alias your columns.. This is also why you shouldn't just pull all columns out but the specific ones you need..
SELECT t1.ID as t1_id,
t1.var1 as t1_var1,
t2.ID as t2_id,
t2.var1 as t2_var1,
... Etc.
FROM t1
LEFT JOIN t2 on t1.ID = t2.ID
With two ID columns and no way to distinguish between the two an error will occur
You could qualify the columns like
SELECT t1.*, t2.* FROM t1 LEFT JOIN t2 ON (t1.ID=t2.ID);
But you should not do it unless the columns with identical names do actually hold/reference the same data.
try like following. this may help you
SELECT t1.*, t2.YourDesiredColumnName FROM t1 LEFT JOIN t2 ON (t1.ID=t2.ID);
I have two tables,
table1 and table2 both tables has these columns
id, name, rel_id
now i would like to have a query to count ids of the table1 where name from table2 is equals to john and table1 rel_id equals to table2 rel_id.
so something like this (this is not correct that's why i need help to make it work).
Select count(ids) from table1
where table2.name="john"
and table1.rel_id=table2.rel_id
Well, one way is to use a join:
Select count(t1.id)
from table1 t1 join
table2 t2
on t1.rel_id = t2.rel_id
where t2.name = 'john';
Note that this uses table aliases to distinguish all the columns in each table. Because the tables have the same columns, you need to identify the table for each column. Also, the I changed the string constant to use single quotes rather than double quotes.
You need to look into joins so :
select count(ids)
from table1 join table2 on table1.rel_id=table2.rel_i
where table2.name="john"
A short intro from W3C schools: http://www.w3schools.com/sql/sql_join.asp
The full MySQL URL for more reference http://dev.mysql.com/doc/refman/5.0/en/join.html
Imagine I have table1 which has a column named 'table_name'. I use table1.table_name to store the name of another table in the database. The referenceable tables would all have a field 'target_id.
Is is possible to use table_name in a JOIN statement?
For example:
SELECT t1.*, t2.* FROM table1 AS t1
JOIN table1.table_name AS t2 ON t1.table1_id = t2.target_id
The obvious solution is to use the script (C++ in my case) to get the table name first, and construct a SQL query from it. The question is: can we bypass the script and do this directly in SQL (MySQL)?
Edit: What is dynamic SQL?
The only chance you have is to do 2 SQL statements:
select the tablename you need
use this table-name to dynamically build the secound query to get the data you need - what you want isn't possible to do with SQL directly (and it sounds like you've designed your database wrong in some way - but that's hard to say without knowing what's the goal of it).
I know I'm late to the party, but I wanted to offer a different solution. I see this sort of thing a lot in audit tables. The column table_name would refer to "what table was changed" and table1_id would refer to the ID of the row that changed in that table. In this case, the audit table is pointing back to many different tables that don't normally get joined.
Here goes:
SELECT t1.*, t2.*, t3.*, t4.*, t5.*
FROM table1 AS t1
left JOIN table2 AS t2
ON t1.table1_id = t2.target_id
and t1.table_name = 'table2'
left JOIN table3 AS t3
ON t1.table1_id = t3.target_id
and t1.table_name = 'table3'
left JOIN table4 AS t4
ON t1.table1_id = t4.target_id
and t1.table_name = 'table4'
left JOIN table5 AS t5
ON t1.table1_id = t5.target_id
and t1.table_name = 'table5'
Of course, the main drawback is that each table that can be possibly referenced needs to be explicitly included in the SQL command.
You can get more elegant output using this as your select list:
SELECT
t1.*,
coalesce(t2.fieldA, t3.fieldA, t4.fieldA, t5.fieldA) as fieldA,
coalesce(t2.fieldB, t3.fieldB, t4.fieldB, t5.fieldB) as fieldB
etc