计算Sql中的行数

Posted 2021-02-22

count(*args)
Count operates using three different approaches.
Count all: By not passing any parameters to count, it will return a count of all the rows for the model.
Count using column: By passing a column name to count, it will return a count of all the rows for the model with supplied column present
Count using options will find the row count matched by the options used.
The third approach, count using options, accepts an option hash as the only parameter. The options are:
:conditions: An SQL fragment like "administrator = 1" or [ "user_name = ?", username ]. See conditions in the intro to ActiveRecord::Base.
:joins: Either an SQL fragment for additional joins like "LEFT JOIN comments ON comments.post_id = id" (rarely needed) or named associations in the same form used for the :include option, which will perform an INNER JOIN on the associated table(s). If the value is a string, then the records will be returned read-only since they will have attributes that do not correspond to the table‘s columns. Pass :readonly => false to override.
:include: Named associations that should be loaded alongside using LEFT OUTER JOINs. The symbols named refer to already defined associations. When using named associations, count returns the number of DISTINCT items for the model you‘re counting. See eager loading under Associations.
:order: An SQL fragment like "created_at DESC, name" (really only used with GROUP BY calculations).
:group: An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.
:select: By default, this is * as in SELECT * FROM, but can be changed if you, for example, want to do a join but not include the joined columns.
:distinct: Set this to true to make this a distinct calculation, such as SELECT COUNT(DISTINCT posts.id) …
:from - By default, this is the table name of the class, but can be changed to an alternate table name (or even the name of a database view).

Person.count(:conditions => "age > 26")