relational-database

问题 Here is the problem : I have a model with 3 classes person person_job job A person can have several jobs, any job-person relation can have a "date_start" attribute, "date_end", and "comment". So I built this model with a jointable (person_job) holding these attributes, and making the relationship on 2 manyToOne attributes called person and job (generated with doctrine annotations) Person attributes looks like : /** * @var string * @ORM\Column(name="name",type="string",length=255,nullable

问题 Have three entities: Project Employee Employment Problem description: Employee can work on many projects and for each he has one employment. I want to have access to all projects and your referred employments of a certain employee. I'm not sure but the relationship must look like a ternary : The physical table is not defined yet. So, be free to design (most basic) them. And my question: How i can build using Laravel Eloquent Relationships ? 回答1: Basically your four tables will be something

问题 Have three entities: Project Employee Employment Problem description: Employee can work on many projects and for each he has one employment. I want to have access to all projects and your referred employments of a certain employee. I'm not sure but the relationship must look like a ternary : The physical table is not defined yet. So, be free to design (most basic) them. And my question: How i can build using Laravel Eloquent Relationships ? 回答1: Basically your four tables will be something

问题 In High Performance Java Persistence book's 6.3.3.3 section it's written that Lost Update phenomena is possible in MySQL Repeatable Read isolation level. This is the screenshot: Assuming the following(isolation level is REPEATABLE READ): tx1 | tx2 ----------------------------------------------------------------------------------- START TRANSACTION; | SELECT * FROM test WHERE id = 1; | ( say, DB_TRX_ID = 7 at this moment) | | | START TRANSACTION; | SELECT * FROM test WHERE id = 1; | UPDATE

问题 I have a question about database relationship I am trying to build a monitoring system with the following rules : Channels belongs to one Sensor Sensors belongs to one Device Devices belongs to one Probe Probes belongs to one Core Here is a preview of the tables +-------------+ +-------------+ | Cores | | Probes | +-------------+ +-------------+ | id | | id | | fields ... | | fields ... | +-------------+ | core_id | +-------------+ +-------------+ +-------------+ +-------------+ | Devices | |

问题 I have a question about database relationship I am trying to build a monitoring system with the following rules : Channels belongs to one Sensor Sensors belongs to one Device Devices belongs to one Probe Probes belongs to one Core Here is a preview of the tables +-------------+ +-------------+ | Cores | | Probes | +-------------+ +-------------+ | id | | id | | fields ... | | fields ... | +-------------+ | core_id | +-------------+ +-------------+ +-------------+ +-------------+ | Devices | |

问题 A task that I'm working on to learn relational algebra is asking me to find the clients who want to rent a property for free, or have not given information about how much they are willing to pay. the table looks like this: Client(clientNo, firstName, lastName, maxRent) I have some trouble finding any information about how to represent a lack of information, or an empty data point.. so far I've come up with this where half of the expression is psuedo code: σ maxRent = 0 OR [no info given]

问题 A task that I'm working on to learn relational algebra is asking me to find the clients who want to rent a property for free, or have not given information about how much they are willing to pay. the table looks like this: Client(clientNo, firstName, lastName, maxRent) I have some trouble finding any information about how to represent a lack of information, or an empty data point.. so far I've come up with this where half of the expression is psuedo code: σ maxRent = 0 OR [no info given]

问题 I was getting a little confused with the last post so I found a nice example which should clear things up. hireDate & carReg are the primary keys. So my question can anyone find any extra functional dependencies other than the ones I have identified below....Modifications also welcome: fd1 carReg -> make, model, outletNo, outletLoc fd2 custNo -> custName fd3 outletNo -> outletLoc fd4 model -> make (only if we assume a model name is unique to a make) fd5 carReg, hireDate -> make, model, custNo

问题 I was getting a little confused with the last post so I found a nice example which should clear things up. hireDate & carReg are the primary keys. So my question can anyone find any extra functional dependencies other than the ones I have identified below....Modifications also welcome: fd1 carReg -> make, model, outletNo, outletLoc fd2 custNo -> custName fd3 outletNo -> outletLoc fd4 model -> make (only if we assume a model name is unique to a make) fd5 carReg, hireDate -> make, model, custNo