diff options
Diffstat (limited to 'Docs/manual.texi')
| -rw-r--r-- | Docs/manual.texi | 14 |
1 files changed, 12 insertions, 2 deletions
diff --git a/Docs/manual.texi b/Docs/manual.texi index 154353b2063..f315bd3652f 100644 --- a/Docs/manual.texi +++ b/Docs/manual.texi @@ -17919,6 +17919,7 @@ Returns the bitwise @code{AND} of all bits in @code{expr}. The calculation is performed with 64-bit (@code{BIGINT}) precision. @end table +@cindex @code{GROUP BY}, extensions to ANSI SQL @strong{MySQL} has extended the use of @code{GROUP BY}. You can use columns or calculations in the @code{SELECT} expressions that don't appear in the @code{GROUP BY} part. This stands for @emph{any possible value for this @@ -17937,8 +17938,9 @@ In ANSI SQL, you would have to add @code{customer.name} to the @code{GROUP BY} clause. In @strong{MySQL}, the name is redundant if you don't run in ANSI mode. -Don't use this feature if the columns you omit from the @code{GROUP BY} part -aren't unique in the group! +@strong{Don't use this feature} if the columns you omit from the +@code{GROUP BY} part aren't unique in the group! You will get +unpredictable results. In some cases, you can use @code{MIN()} and @code{MAX()} to obtain a specific column value even if it isn't unique. The following gives the value of @@ -19292,6 +19294,7 @@ to tell the optimizer that the result set will have many rows. In this case, temporary table with a key on the @code{GROUP BY} elements. @item +@cindex @code{GROUP BY}, extensions to ANSI SQL If you use @code{GROUP BY}, the output rows will be sorted according to the @code{GROUP BY} as if you would have had an @code{ORDER BY} over all the fields in the @code{GROUP BY}. @strong{MySQL} has extended the @code{GROUP BY} so that @@ -19302,6 +19305,13 @@ SELECT a,COUNT(b) FROM test_table GROUP BY a DESC @end example @item +@strong{MySQL} has extended the use of @code{GROUP BY} to allow you to +select fields which are not mentioned in the @code{GROUP BY} clause. +If you are not getting the results you expect from your query, please +read the @code{GROUP BY} description. +@xref{Group by functions}. + +@item @code{SQL_BUFFER_RESULT} will force the result to be put into a temporary table. This will help @strong{MySQL} free the table locks early and will help in cases where it takes a long time to send the result set to the client. |