What does it mean by select 1 from table

Question

I have seen many queries with something as follows   Select 1   From table   What does this 1 mean  how will it be executed and  what will it return   Also  in what type of scenarios  can this be used

User · Answer

The reason is another one  at least for MySQL  This is from the MySQL manual     InnoDB computes index cardinality values for a table the first time that table is accessed after startup  instead of storing such values in the table  This step can take significant time on systems that partition the data into many tables  Since this overhead only applies to the initial table open operation  to    warm up    a table for later use  access it immediately after startup by issuing a statement such as SELECT 1 FROM tbl name LIMIT 1

User · Answer

it does what it says - it will always return the integer 1  It s used to check whether a record matching your where clause exists

User · Answer

The result is 1 for every record in the table

User · Answer

If you just want to check a true or false based on the WHERE clause  select 1 from table where condition is the cheapest way

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SELECT 1 FROM TABLE NAME means   Return 1 from the table   It is pretty unremarkable on its own  so normally it will be used with WHERE and often EXISTS  as  gbn notes  this is not necessarily best practice  it is  however  common enough to be noted  even if it isn t really meaningful  that said  I will use it because others use it and it is  more obvious  immediately  Of course  that might be a viscous chicken vs  egg issue  but I don t generally dwell      SELECT   FROM TABLE1 T1 WHERE EXISTS        SELECT 1 FROM TABLE2 T2 WHERE T1 ID  T2 ID       Basically  the above will return everything from table 1 which has a corresponding ID from table 2   This is a contrived example  obviously  but I believe it conveys the idea  Personally  I would probably do the above as SELECT   FROM TABLE1 T1 WHERE ID IN  SELECT ID FROM TABLE2   as I view that as FAR more explicit to the reader unless there were a circumstantially compelling reason not to    EDIT  There actually is one case which I forgot about until just now  In the case where you are trying to determine existence of a value in the database from an outside language  sometimes SELECT 1 FROM TABLE NAME will be used  This does not offer significant benefit over selecting an individual column  but  depending on implementation  it may offer substantial gains over doing a SELECT    simply because it is often the case that the more columns that the DB returns to a language  the larger the data structure  which in turn mean that more time will be taken

User · Answer

If you don t know there exist any data in your table or not  you can use following query    SELECT cons value FROM table name    For an Example   SELECT 1 FROM employee     It will return a column which contains the total number of rows  amp  all rows have the same constant value 1  for this time it returns 1 for all rows   If there is no row in your table it will return nothing    So  we use this SQL query to know if there is any data in the table  amp  the number of rows indicates how many rows exist in this table

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To be slightly more specific  you would use this to do  SELECT 1 FROM MyUserTable WHERE user id   33487   instead of doing  SELECT   FROM MyUserTable WHERE user id   33487   because you don t care about looking at the results  Asking for the number 1 is very easy for the database  since it doesn t have to do any look-ups

User · Answer

If you mean something like  SELECT   FROM AnotherTable   WHERE EXISTS  SELECT 1 FROM table WHERE        then it s a myth that the 1 is better than  SELECT   FROM AnotherTable   WHERE EXISTS  SELECT   FROM table WHERE        The 1 or   in the EXISTS is ignored and you can write this as per Page 191 of the ANSI SQL 1992 Standard   SELECT   FROM AnotherTable   WHERE EXISTS  SELECT 1 0 FROM table WHERE

User · Answer

This is just used for convenience with IF EXISTS    Otherwise you can go with   select   from  table name   Image In the case of  IF EXISTS   we just need know that any row with specified condition exists or not doesn t matter what is content of row   select 1 from Users  above example code  returns no  of rows equals to no  of users with 1 in single column

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I see it is always used in SQL injection such as   www urlxxxxx com xxxx asp id 99 union select 1 2 3 4 5 6 7 8 9 from database   These numbers can be used to guess where the database exists and guess the column name of the database you specified And the values of the tables

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This means that You want a value  1  as output or Most of the time used as Inner Queries because for some reason you want to calculate the outer queries based on the result of inner queries   not all the time you use 1 but you have some specific values     This will statically gives you output as value 1

User · Answer

select 1 from table will return the constant 1 for every row of the table  It s useful when you want to cheaply determine if record matches your where clause and or join

User · Answer

select 1 from table is used by some databases as a query to test a connection to see if it s alive  often used when retrieving or returning a connection to   from a connection pool

User · Answer

it simple means that you are retrieving the number first column from table     means select Emply num Empl no From Employees   here you are using select 1 from Employees   that means you are retrieving the Emply num column  Thanks

User · Answer

Although it is not widely known  a query can have a HAVING clause without a GROUP BY clause    In such circumstances  the HAVING clause is applied to the entire set  Clearly  the SELECT clause cannot refer to any column  otherwise you would  correct  get the error   Column is invalid in select because it is not contained in the GROUP BY  etc    Therefore  a literal value must be used  because SQL doesn t allow a resultset with zero columns -- why    and the literal value 1  INTEGER  is commonly used  if the HAVING clause evaluates TRUE then the resultset will be one row with one column showing the value 1  otherwise you get the empty set   Example  to find whether a column has more than one distinct value   SELECT 1   FROM tableA HAVING MIN colA   lt  MAX colA

[mysql] What does it mean by select 1 from table?

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