ADD PRIMARY KEY (existing_column); Primary Key vs Unique Key in SQL: Description:Both ensure uniqueness, but a Primary Key is used to identify records, and it cannot contain NULL values. Unique Key allows NULL values. Code Example: -- Primary Key CREATE TABLE example_table ( id INT PRIMAR...
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within ...
Sql - Referencing multiple primary keys in another table, create table TEST_TABLE2 /* */ ( PRIMARY_COLUMN1 int references TEST_TABLE1 (PRIMARY_COLUMN1) not null, primary key (PRIMARY_COLUMN1) ); What you have here is one primary key that happens to contain multiple columns (aka. "compo...
If you're doing this through the GUI (SQL Server Management Studio) and the table designer you simply hold shift and click the grey area to the left of the column name & type. Then right-click and choose the option to make a primary key. Presto! ;)...
COLUMN_NAME (ODBC 1.0)4Varchar 非 NULL主键列名称。 驱动程序返回没有名称的列的空字符串。 KEY_SEQ (ODBC 1.0)5Smallint(非 NULL)键 (中的列序列号从 1) 开始。 PK_NAME (ODBC 2.0)6Varchar主键名称。 如果不适用于数据源,则为 NULL。
(just for that session) a value for sql_generate_invisible_primary_key to OFF, drop the column that is a primary key and add the primary key of your choice. Then you can revert that system variable to ON, if you wish. Your request for the change of behaviour with that system variable...
如果 ColumnX 涵蓋在索引鍵中,才會取得索引鍵範圍鎖定。 範例 下列資料表和索引是用來作為索引鍵範圍鎖定範例要遵循的基礎。 範圍掃描查詢 為了確保範圍掃描查詢是可序列化,相同的查詢每次在相同交易內執行時都必須傳回相同的結果。 其他的交易絕不能把新的資料列插入範圍掃描查詢內;否則這些動作將會變成虛設項目插入...
check for the existence of multiple columns in a table Check for valid Ip address Check if amount is positive or negative Check If Column Is PRIMARY KEY Check if database exists? Check if Feb-29 is falling between start and end dates Check if file exists then delete it- fix code Check ...
PRIMARY KEY(key) 唯一索引 不可以出现相同的值,可以有NULL值 UNIQUE(key) 主键索引和唯一索引的差别: 唯一索引可以有一个NULL值,只要整体不重复就行了。而主键索引,是非空唯一索引。 普通索引 允许出现相同的索引内容 INDEX(key) -- OR KEY(key[,...]) ...
1-Column Attributes 列属性 2-Inserting a Row 插入单行 / INSERT INTO … VALUES () 3-Inserting Multiple Rows 插入多行 INSERT INTO … VALUES (), (), () 4-Inserting Hierarchical Rows 插入分层行 5-Creating a Copy of a Table 创建表复制 / CREATE TABLE … AS 6-Updating a Single Row 更新...