第一步:删除表约束,清空表,写入测试数据 test=#altertabletbl_uniquedropconstraintuk_tbl_unique_a_b ;ALTERTABLEtest=#deletefromtbl_unique ;DELETE0test=#insertintotbl_unique (a,b)values(1,1),(1,1),(1,1);INSERT03test=#insertintotbl_unique (a)values(2),(2),(2);INSERT03test=#selectoid,...
How Does UNIQUE Constraint Work in PostgreSQL? Each time when you insert a new record in a table, the UNIQUE constraint checks whether the value to be inserted already exists in the table or not. If the specified value already exists, then the UNIQUE constraint generates an error stating that...
The UNIQUE constraint in PostgreSQL violated when more than one row for a column or combination of columns which have been used as a unique constraint in a table. Two NULL values for a column in different rows is different and it does not violate the uniqueness of UNIQUE constraint. When a...
第五步:增加唯一键约束 test=# alter table tbl_unique add constraint uk_tbl_unique_a_b unique (a,b); ALTER TABLE 1. 2. 情况四:将非严格意义上重复数据删除到只有一条 第一步:删除唯一约束,清空表,写入测试数据 test=# alter table tbl_unique drop constraint uk_tbl_unique_a_b ; ALTER TABLE ...
Adding a unique constraint willautomatically create a unique B-tree indexon the column or group of columns listed in the constraint. A uniqueness restriction covering only some rows cannot be written as a unique constraint, but it is possible to enforce such a restriction by creating a uniquepar...
Summary: in this tutorial, you will learn about PostgreSQL UNIQUE constraint to make sure that values stored in a column or a group of columns are unique across rows in a table. Introduction to PostgreSQL UNIQUE constraint Sometimes, you want to ensure that values stored in a column or a gr...
--PostgreSQL 9.6 2 --'\\' is a delimiter 3 4 selectversion()aspostgresql_version; 5 6 7 createtableaa (aintprimarykey, bint); 8 createtablebb (cint, dint,UNIQUE(c, d));--c + d combination must be unique
“postgresql duplicate key value violates unique constraint”错误的含义 在PostgreSQL中,“duplicate key value violates unique constraint”错误意味着你尝试插入或更新数据库中的数据,而该数据违反了已存在的唯一约束(unique constraint)。唯一约束是一种数据库完整性约束,它确保一列或多列的组合在表中是唯一的,不允...
CREATETABLEnull_old_style(idBIGINTGENERATEDBYDEFAULTASIDENTITYPRIMARYKEY,val1TEXTNOTNULL,val2TEXTNULL,CONSTRAINTuq_val1_val2UNIQUE(val1,val2)); null_new_style表使用新的选项:UNIQUE NULLS NOT DISTINCT。和上面的表唯一区别就是唯一约束的新语法: ...
In PostgreSQL, the “UNIQUE” keyword is used with the CREATE TABLE or ALTER TABLE commands to add a unique constraint on single/multiple columns of a new or an already existing Postgres table. Moreover, PostgreSQL uses the ALTER TABLE command with the DROP CONSTRAINT clause to...