Is there a difference between Select * and Select [list each col] [duplicate]

Answer 1

1) The second one is more explicit about which columns are returned. The value of the 2nd one then is how much you value explicitly knowing which columns come back.

2) This involves potentially less data being returned when there are more columns than the ones explicitly used as well.

3) If you change the table by adding a new column, the first query changes and the second does not. If you have code like "for all columns returned do ..." then the results change if you use the first, but not the 2nd.

Answer 2

It depends on what you mean by "difference". There is the obvious syntax difference, but the real difference is one of performance.

When you say SELECT * FROM MyTable, you are telling the SQL query engine to return a data set with all of the columns from that table, while SELECT ColA, ColB, ColC FROM MyTable tells the query engine to return a data set with only ColA, ColB, and ColC from the table.

Say you have a table with 100 columns defined as CHAR[10]. SELECT * will return 100 columns * 10 bytes worth of data while SELECT ColA, ColB, ColC will return 3 columns * 10 bytes worth of data. This is a huge size difference in the amount of data that is being passed back across the wire.

Specifying the column list also makes it much clearer what columns you are interested in. The drawback is that if you add/remove a column from the table you need to ensure that the column list is updated as well, but I think that's a small price compared to the performance gain.

Answer 3

SELECT * FROM MyTable

select * is dependent on the column order in the schema so if you refer to the result set by the index # of the collection you will be looking at the wrong column.

SELECT Col1,Col2,Col3 FROM MyTable

this query will give you a collection that stays the same over time, but how often are you changing the column order anyways?

Answer 4

You should specify an explicit column list. SELECT * will bring back more columns than you need creating more IO and network traffic, but more importantly it might require extra lookups even though a non-clustered covering index exists (On SQL Server).

Answer 5

Using "SELECT *" optimizes for programmer typing. That's it. That's the only advantage.

Answer 6

When you select each field individually, it is more clear which fields are actually being selected.