ALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]    action [, ... ]ALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]    RENAME [ COLUMN ] column_name TO new_column_nameALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]    RENAME CONSTRAINT constraint_name TO new_constraint_nameALTER TABLE [ IF EXISTS ] name    RENAME TO new_nameALTER TABLE [ IF EXISTS ] name    SET SCHEMA new_schemaALTER TABLE ALL IN TABLESPACE name [ OWNED BY role_name [, ... ] ]    SET TABLESPACE new_tablespace [ NOWAIT ]where action is one of:    ADD [ COLUMN ] [ IF NOT EXISTS ] column_name data_type [ COLLATE collation ] [ column_constraint [ ... ] ]    DROP [ COLUMN ] [ IF EXISTS ] column_name [ RESTRICT | CASCADE ]    ALTER [ COLUMN ] column_name [ SET DATA ] TYPE data_type [ COLLATE collation ] [ USING expression ]    ALTER [ COLUMN ] column_name SET DEFAULT expression    ALTER [ COLUMN ] column_name DROP DEFAULT    ALTER [ COLUMN ] column_name { SET | DROP } NOT NULL    ALTER [ COLUMN ] column_name SET STATISTICS integer    ALTER [ COLUMN ] column_name SET ( attribute_option = value [, ... ] )    ALTER [ COLUMN ] column_name RESET ( attribute_option [, ... ] )    ALTER [ COLUMN ] column_name SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }    ADD table_constraint [ NOT VALID ]    ADD table_constraint_using_index    ALTER CONSTRAINT constraint_name [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]    VALIDATE CONSTRAINT constraint_name    DROP CONSTRAINT [ IF EXISTS ]  constraint_name [ RESTRICT | CASCADE ]    DISABLE TRIGGER [ trigger_name | ALL | USER ]    ENABLE TRIGGER [ trigger_name | ALL | USER ]    ENABLE REPLICA TRIGGER trigger_name    ENABLE ALWAYS TRIGGER trigger_name    DISABLE RULE rewrite_rule_name    ENABLE RULE rewrite_rule_name    ENABLE REPLICA RULE rewrite_rule_name    ENABLE ALWAYS RULE rewrite_rule_name    DISABLE ROW LEVEL SECURITY    ENABLE ROW LEVEL SECURITY    FORCE ROW LEVEL SECURITY    NO FORCE ROW LEVEL SECURITY    CLUSTER ON index_name    SET WITHOUT CLUSTER    SET WITH OIDS    SET WITHOUT OIDS    SET TABLESPACE new_tablespace    SET { LOGGED | UNLOGGED }    SET ( storage_parameter = value [, ... ] )    RESET ( storage_parameter [, ... ] )    INHERIT parent_table    NO INHERIT parent_table    OF type_name    NOT OF    OWNER TO { new_owner | CURRENT_USER | SESSION_USER }    REPLICA IDENTITY { DEFAULT | USING INDEX index_name | FULL | NOTHING }and table_constraint_using_index is:    [ CONSTRAINT constraint_name ]    { UNIQUE | PRIMARY KEY } USING INDEX index_name    [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE 

 PostgreSQL: Documentation: 9.6: ALTER TABLE

例子：

alter table ask add column  commentCount int not null default 0;

mew=# drop type gradetype cascade;
注意: 递归删除 表 users 字段 grade
DROP TYPE

这样级联删除类型会将有这个类型的表的对应字段删除

改表 alter table