ALTER TABLE students DROP CONSTRAINT unique_email; (3)添加非空约束: ALTER TABLE 表名 ALTER COLUMN 字段名 SET NOT NULL; 示例: ALTER TABLE students ALTER COLUMN student_name SET NOT NULL; (4)删除非空约束: ALTER TABLE 表名 ALTER COLUMN 字段名 DROP NOT NULL; 示例: ALTER TABLE students ALTER...
In PostgreSQL, the constraints are used to apply some rules on the table’s column. In Postgres, theNOT NULLconstraint prevents NULL entries from being inserted into a column. In simple terms, the table columns declared with aNOT NULLconstraint take only non-null entries. In Postgres, theNOT...
/*test=# create table tbl_check(a int not null check (a>0),b varchar(12) not null check (b in ('ab','Ab','aB','AB'))); CREATE TABLE test=# drop table tbl_check ; DROP TABLE test=# create table tbl_check test-# ( test(# a int not null, test(# b varchar(12) not ...
ALTER DOMAIN name { SET DEFAULT expression | DROP DEFAULT } ALTER DOMAIN name { SET | DROP } NOT NULL ALTER DOMAIN name ADD domain_constraint ALTER DOMAIN name DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ] ALTER DOMAIN name OWNER TO new_owner ALTER FUNCTION 修改一个函数的定义。 ALT...
ALTERTABLEtable_nameALTERCOLUMNcolumn_nameDROPNOTNULL; 以下语句将会删除产品表name 字段上的非空约束: test=#ALTERTABLEproductsALTERCOLUMNnameDROPNOTNULL;ALTERTABLEtest=# \d products;Table"hr.products"Column|Type|Collation|Nullable|Default---+---+---+---+---product_no|integer||notnull|name|text...
drop table guess_what;create tableguess_what(id serial primary key,first_namevarchar(20),last_namevarchar(20),box1varchar(10),box2varchar(10),box3varchar(10),date_time timestamp without time zoneDEFAULTnow());insert intoguess_what(first_name,last_name,box2)values('Simon','Almbo','box2...
1.设置NOT NULL约束的字段INSERT必须赋值,没有NOT NULL约束的字段INSERT没有赋值,会自动填充NULL。 /* postgres=# create database test with template = template0 encoding='UTF8' lc_collate='C' lc_ctype='C'; CREATE DATABASE postgres=# postgres=# ...
一个索引可以使用PostgreSQL的 DROP 命令删除。 代码语言:javascript 代码运行次数:0 运行 AI代码解释 DROPINDEXindex_name; 您可以使用下面的语句来删除之前创建的索引: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 #DROPINDEXsalary_index; 删除后,可以看到 salary_index 已经在索引的列表中被删除: ...
3.删除NOT NULL约束 /* test=# alter table tbl_null alter COLUMN b drop not null;ALTER TABLE test=# \d tbl_null Table "public.tbl_null"Column | Type | Modifiers ---+---+--- a | integer | not null b | character varying(12) | */ ⼆、DEFAULT --- 默认值 ...